Added documentation on adding database indexes

## What does this MR do?

This MR adds a guide on when to add database indexes, how to approach this problem, etc

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

Spelling, grammar, etc

## Why was this MR needed?

Blindly adding indexes can be harmful to the database, and I recently had to remove 48-something unused indexes.

## What are the relevant issue numbers?

#20767

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] All builds are passing
- [ ] 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 you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5771

- Use assertions in the vein of `merge_access_levels.map(&:access_level)`
  instead of `merge_access_levels.first.access_level`

Revert "Merge branch '19957-write-tests-for-adding-comments-for-different-line-types-in-diff' into 'master'"

This reverts merge request !5417

Add reference to product map.

## What does this MR do?
Mention the product map in the UI Guide and remove a line that says that the GitLab logo and user picture and in the sidebar.

## What are the relevant issue numbers?
19350

- [ ] [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 [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
Closes #19350

See merge request !5769

- `#find_users` depends on a project being loaded.

- Missed adding this in 2193ae22

- This is an optimization that was made in !581, and it needs to be
  backported to CE to avoid merge conflicts in the future.

- Previously created a protected branch manually, without using a
  factory.

1. `GitPushService` was still using `{merge,push}_access_level_attributes` instead
   of `{merge,push}_access_levels_attributes`.

2. The branches API creates access levels regardless of the state of the
   `developers_can_{push,merge}` parameters. This is in line with the UI,
   where Master access is the default for a new protected branch.

3. Use `after(:build)` to create access levels in the
   `protected_branches` factory, so that `factories_spec` passes. It
   only builds records, so we need to create access levels on `build` as
   well.

1. Previously, we were using `after_create` to create access levels.

2. At the time of protected branch creation, there are _no_ access
   levels present, which is invalid, and creation fails.

3. Fixed by setting access levels before the protected branch is created.

1. Change a few incorrect `access_level` to `access_levels.first` that
   were missed in e805a647.

2. `API::Entities` can iterate over all access levels instead of just
   the first one. This makes no difference to CE, and makes it more compatible
   with EE.

1. To improve EE compatibility.

1. This is in regard to the protected branches feature spec.

2. For example, if "Masters" is already selected, don't re-select
   "Masters" during the spec.

3. This is due to a bug in the frontend implementation, where selecting
   an already-selected access level _deselects_ it, which is something
   we don't need. I'll create a separate issue for this.

4. This hasn't turned up before, because we were manually creating
   missing access levels prior to e805a647. Now, we just use nested
   attributes, and missing access levels fail validation.

!581 has a lot of changes that would cause merge conflicts if not
properly backported to CE. This commit/MR serves as a better
foundation for gitlab-org/gitlab-ee!581.

= Changes =

1. Move from `has_one {merge,push}_access_level` to `has_many`, with the
   `length` of the association limited to `1`. This is _effectively_ a
   `has_one` association, but should cause less conflicts with EE, which
   is set to `has_many`. This has a number of related changes in the
   views, specs, and factories.

2. Make `gon` variable loading more consistent (with EE!581) in the
   `ProtectedBranchesController`. Also use `::` to prefix the
   `ProtectedBranches` services, because this is required in EE.

3. Extract a `ProtectedBranchAccess` concern from the two access level
   models. This concern only has a single `humanize` method here, but
   will have more methods in EE.

4. Add `form_errors` to the protected branches creation form. This is
   not strictly required for EE compatibility, but was an oversight
   nonetheless.

Upgrade httpclient gem from 2.7.0.1 to 2.8.2.

Fixes deprecation warnings from Ruby 2.3. Resolves #20950.

Changelog: https://github.com/nahi/httpclient/blob/b51d7a8bb78f71726b08fbda5abfb900d627569f/CHANGELOG.md#changes-in-282

cc: @stanhu 

See merge request !5816

Submit to Akismet Part 1 (Issues)

Related to #5932 #5573 gitlab-com/infrastructure#14

See merge request !5538

Add rspec feature tests for the diff notes feature.

## What are the relevant issue numbers?

Closes #19957.

See merge request !5417

Fixes deprecation warnings from Ruby 2.3.

Changelog: https://github.com/nahi/httpclient/blob/b51d7a8bb78f71726b08fbda5abfb900d627569f/CHANGELOG.md#changes-in-282

This fixes a long running tests due to changed Sidekiq state

## What does this MR do?
Fixes a ordering of tests problem, where GroupsController tests change global state and this affects IrkerService

cc @connorshea 

See merge request !5819

Allow to sort by project size on admin projects page

Fixes #20317

See merge request !5797

Fix attribute inclusion import/export config ignored in some cases

In the `import_export.yml` file we define the inclusion of some of the attributes. For some reason, this isn't working in certain cases - very unfortunate this includes `user`. This has been introduced in 8.10.3.

Related https://gitlab.com/gitlab-org/gitlab-ce/issues/20802

See merge request !1982

Set permissions to admin for importing a project via Import/Export

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20802

In order to import a project, it is now required to be an admin

Moved from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5766

See merge request !1983

Add test coverage badge

## What does this MR do?

This MR adds a test coverage badge.

Test coverage badge will be a SVG image you can embed in README.md or on your website that will be created dynamically by GitLab, to show information about how much of the code is being covered by tests. In addition to coverage-regexp feature we currently have, this feature will make it possible to use the coverage data obtained from the build log. We will support both - pipeline coverage, or test coverage for particular job in the pipeline. 

![coverage_badges](/uploads/dfa307339eb58c8138e551b42c0d8756/coverage_badges.png)

![coverage_badge](/uploads/30406e4bdcf979b2900ffe8996728c97/coverage_badge.png)

## What are the relevant issue numbers?

Closes #3714

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [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 you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5708

Fix pipeline and build seeds in development environment

## What does this MR do?

When we depend on state machine events in seeds, it is likely that we will break fixtures from time to time because when transition rules change, using events most likely invalidates some objects in seeds

## Why was this MR needed?

Pipeline and build seeds were broken in development environment.

## What are the relevant issue numbers?

Closes #20933

See merge request !5810

When we depend on state machine events in seeds, it is likely that we
will break fixtures from time to time because when transition rules
change, using events most likely invalidates some objects in seeds.

- Refactored SpamCheckService into SpamService

- Added controller actions as reusable concerns
- Added controller tests

- Added `submitted_as_ham` to `SpamLog` to mark which logs have been submitted to Akismet.
- Added routes and controller action.

- Removed unnecessary column from `SpamLog`
- Moved creation of SpamLogs out of its own service and into SpamCheckService
- Simplified code in SpamCheckService.
- Moved move spam related code into Spammable concern

- Merged `AkismetSubmittable` into `Spammable`
- Clean up `SpamCheckService`
- Added tests for `Spammable`
- Added submit (ham or spam) options to `AkismetHelper`

- New concern `AkismetSubmittable` to allow issues and other `Spammable` models to be submitted to Akismet.
- New model `UserAgentDetail` to store information needed for Akismet.
- Services needed for their creation and tests.

Add method missing from EE

## Why was this MR needed?

`ProjectWiki#repository_exists?` is present in EE but not on CE, and there's no particular reason why it shouldn't

See merge request !5802

Show deployment status on a MR view

## What are the relevant issue numbers?

Resolves #19571, one in the list of #19992

## Screenshots (if relevant)

external_url = nil
![Screen_Shot_2016-08-02_at_13.57.03](/uploads/20ea1587eea556c7a1acd0ff726a5bfb/Screen_Shot_2016-08-02_at_13.57.03.png)

external_url != nil
![Screen_Shot_2016-08-02_at_13.59.59](/uploads/0094b9ddece3f4bf76c83988840c096d/Screen_Shot_2016-08-02_at_13.59.59.png)

Note, the timings are weird between merging and deploying, that is because I did it in the wrong order.

## Does this MR meet the acceptance criteria?

- [X] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing

See merge request !5622