Add an API styleguide

This adds a styleguide for API development. Spinned-of from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6860#note_16922017.

[ci skip]

See merge request !6864

Adds test for feature in this MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6720

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6720

See merge request !6854

Enable SingleLinePerSelector in scss-lint

## What does this MR do?
Enables `SingleLinePerSelector` in `.scss-lint.yml`

## Are there points in the code the reviewer needs to double check?
This MR touches a lot of files, just need another pair of eyes to look through and make sure I didn't break anything.

## Why was this MR needed?
Improve SCSS maintainability and consistency

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#23485

See merge request !7081

Stop unauthorized users dragging on issue boards

Closes #23763

See merge request !7096

Closes #23763

Change the approach to check if patches apply cleanly

Closes #23372

See merge request !6949

Show commits from source project. Be consistent with pipeline

## What does this MR do?

Show commits from source project instead of target project.

## Why was this MR needed?

To be consistent.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] Added for this feature/bug

## What are the relevant issue numbers?

Closes #3596

See merge request !6658

Fix branch protection API.

## What does this MR do?

- Fixes the branch protection API.
- Closes #21513 
- EE Merge Request: gitlab-org/gitlab-ee!718

## Tasks

- [ ]  #21513 !6215 Protected branches API bug
    - [x]  Investigate
    - [x]  Test + Fix
    - [x]  Changelog
    - [x]  MR
    - [x]  Wait for build to pass
    - [x]  Review
    - [x]  Check for EE conflicts
    - [x]  Create EE MR
    - [x]  Refactor + Fix
    - [x]  Rebase EE MR against EE master
    - [x]  Wait for builds to pass
    - [x]  Assign to dbalexandre/douwe
    - [x]  Implement latest review comments
    - [x]  Wait for Douwe's review
        - [x]  Implement changes
        - [x]  Port changes to EE MR
        - [x]  Assign both back to Douwe
    - [ ]  Wait for merge
    - [ ]  Merge gitlab-org/gitlab-ee!718

See merge request !6215

Fix `User#to_reference`

## What does this MR do?

Fix the method signature of `User#to_reference` so that moving an issue with a user reference does not throw a "invalid number of arguments" exception.

## Why was this MR needed?

1. Changes in 8.13 require `Referable`s that don't have a project
   reference to accept two arguments - `from_project` and
   `target_project`.

2. `User#to_reference` was not changed to accept the
   `target_project` (even though it is not used). Moving an issue
   containing a user reference would throw a "invalid number of
   arguments" exception.

3. The regression was introduced in [c8b2b3f7](https://gitlab.com/gitlab-org/gitlab-ce/commit/c8b2b3f7c32db873f1bebce3e3b1847ea24d235f#91fabb7ad88bd2fde6fef1c100a719c00e503047_75_79), which expects
   all `Referable`s that don't respond to `:project` to have a `to_reference`
   method that takes two arguments.   

## Does this MR meet the acceptance criteria?

- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

- Closes #23662

See merge request !7088

Escape `ref` and `path` in `RelativeLinkFilter`.

fixes #21420

See merge request !6050

Add a note regarding syncing Git submodules in CI

Replaces https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7047

See merge request !7095

[ci skip]

Stop injecting field errors where they won't be used.

## What does this MR do?

Filters the form elements which gl_field_errors validates, excluding input types button, submit and checkbox.

## Why was this MR needed?

This won't make a difference in UX, but I just noticed that we are currently validating and injecting errors into the DOM for all non-hidden inputs. Doing so is unnecessary, and could introduce performance problems on forms with larger numbers of inputs.

## Does this MR meet the acceptance criteria?

- [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6929

Add docker-compose environment initialization command

## What does this MR do?

Updates a snippet for setting up koding docker-compose environment in the integration document.

See merge request !7091

Fix bug where labels would be assigned to issues that were moved

If you attempt to move an issue from one project to another and leave
labels blank, LabelsFinder would assign all labels in the new project
to that issue. The issue is that :title is passed along to the Finder,
but since it appears empty no filtering is done. As a result, all
labels in the group are returned. This fix handles that case.

Closes #23668

See merge request !7065

Change overflow scroll to auto

Changes overflow scroll to auto

Closes #23467

See merge request !7048

Remove redundant overrides in ActiveModel association declarations.

Closes #23509

See merge request !6970

Signed-off-by: Sonmez Kartal <sonmez@koding.com>

If you attempt to move an issue from one project to another and leave
labels blank, LabelsFinder would assign all labels in the new project
to that issue. The issue is that :title is passed along to the Finder,
but since it appears empty no filtering is done. As a result, all
labels in the group are returned. This fix handles that case.

Closes #23668

1. Changes in 8.13 require `Referable`s that don't have a project
   reference to accept two arguments - `from_project` and
   `target_project`.

2. `User#to_reference` was not changed to accept the
   `target_project` (even though it is not used). Moving an issue
   containing a user reference would throw a "invalid number of
   arguments" exception.

Fixes #23662

Only show register tab if signup enabled.

## What does this MR do?

Fixes a regression where the register tab is activated, even if sign-up enabled is not activated in application_settings.

## Screenshots (if relevant)

When signup is disabled: 

![Screen_Shot_2016-10-22_at_8.56.23_PM](/uploads/bd0fa5f27114779c0d290a8151c1c253/Screen_Shot_2016-10-22_at_8.56.23_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23654

See merge request !7058

Enable SpaceAfterVariableColon in scss-lint

## What does this MR do?
Enables `SpaceAfterVariableColon` in `.scss-lint.yml`

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Improve SCSS maintainability and consistency

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#23485

See merge request !7080

Enable SpaceAroundOperator in scss-lint

## What does this MR do?
Enables `SpaceAroundOperator` in .`scss-lint.yml`

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Improve SCSS maintainability and consistency

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#23485

See merge request !7078

They were Rails' default and are unnecessarily overridden.
Signed-off-by: David Wagner <david@marvid.fr>

Enable trailingWhitespace in scss-lint

## What does this MR do?
Enables `trailingWhitespace` in `.scss-lint.yml`

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Improve SCSS maintainability and consistency

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#23485 

See merge request !7077

Fix pipeline graph hidden on commit and mr pages

## What does this MR do?

Dynamically invokes `new gl.Pipelines();` when an MR `builds` tab is clicked.

Dispatches `new gl.Pipelines();` on a `commit#builds` page.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

The pipeline graph was hidden on commit and mr pages

## Screenshots (if relevant)

Commit page:
![Screen_Shot_2016-10-14_at_18.16.18](/uploads/ee11dea0825d1489dc167292e16c8f41/Screen_Shot_2016-10-14_at_18.16.18.png)

MR:
![Screen_Shot_2016-10-14_at_18.16.39](/uploads/602c2fce2397c799bedb757bfd3010af/Screen_Shot_2016-10-14_at_18.16.39.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #23325

See merge request !6895

Signed-off-by: Rémy Coutable <remy@rymai.me>

Resolve "Build sidebar has extra space on Firefox"

## What does this MR do?

Accounts for the deprecated use of `document.body.scrollTop` on FF that was breaking the `translateSidebar` method.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

## Screenshots (if relevant)

![2016-10-22_23.16.31](/uploads/496810a9bb4fa4c3762023ff175572cb/2016-10-22_23.16.31.gif)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #23665

See merge request !7060