Guide about what requires downtime

## What does this MR do?

This MR adds a guide describing various SQL operations and whether they need downtime or not.

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

Spalling and styling of the text mostly.

## Why was this MR needed?

Developers aren't always aware of the impact of certain operations and the documentation of MySQL and PostgreSQL can be quite confusing at times.

## Screenshots (if relevant)

![screenshot](/uploads/d8afd4bd3755d26e4786dfafecfa9368/screenshot.png)

<img src="https://emoji.slack-edge.com/T02592416/trollface/8c0ac4ae98.png" width="60" />

## 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)
- Tests
  - [ ] 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)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5672

Ignore URLs starting with // in Markdown links

## What does this MR do?

Render `[foo](//bar/baz)` as `<a href="//bar/baz">foo</a>`.

## Why was this MR needed?

`[foo](//bar/baz)` currently renders as `<a href="//bar/gitlab-org/gitlab-ce/master/baz">foo</a>`

## What are the relevant issue numbers?

fixes #7032

See merge request !5677

Add issue description to txt/plain emails

## What does this MR do?
Adds issue description to new_issue mail when in text/plain content-type

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

## Why was this MR needed?
Because clients that are rendering emails in text mode can see issue description 

## What are the relevant issue numbers?
#1821 

## Screenshots (if relevant)

## 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)
- [ ] API support added
- Tests
  - [ ] 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 !5663

Add unfold links for Side-by-Side view

## What does this MR do?

Add unfold links for Side-by-Side view and refactor some diff related code

## What are the relevant issue numbers?

Closes #3877 

## 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)
- [x] 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 !5415

Upgrade newrelic_rpm from 3.14.1.311 to 3.16.0.318.

## What does this MR do?

While we intend to remove newrelic in the future, it'll probably not happen until 9.0 due to. In the meantime, I'd like to update the gem so it won't block Rails 5.

Changelog: https://github.com/newrelic/rpm/blob/master/CHANGELOG

## Why was this MR needed?

Rails 5 compatibility.

Relevant issues: #14286, #12860 

See merge request !5553

Move abilities by subject class to a dedicated method

## What does this MR do?

Twin of gitlab-org/gitlab-ee!622.

It moves the `case` that returns abilities depending on the subject class to a dedicated method.

This should avoid lame conflicts when CE to EE since the EE version assign the result of the `case` to a variable.

## What are the relevant issue numbers?

Closes #20621.

See merge request !5668

Add a data migration to fix some missing timestamps in the members table (again)

## What does this MR do?

Repeats an earlier migration to fix historic bad data in the members table (missing created_at and updated_at fields)

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

I'm expecting the WHERE clauses to be fast enough, and to return few enough rows, that the migration doesn't need to use batches, but I'm not too familiar with the size of these tables in the wild, so perhaps that's a poor assumption. 

## Why was this MR needed?

8.10 introduced a dependency on the  `members.created_at` field in the project and namespace member view. If bad data is present, viewing the list of members now results in an NoMethodError and a 500 response from GitLab. Although the previous migration should have fixed all bad rows, we have evidence that it didn't in at least one case, despite the migration claiming to have run in the past.

## What are the relevant issue numbers?

#20568 

## Screenshots (if relevant)

## 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 [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 #20568

See merge request !5670

Set consistency in list text height css

Fix line height of list row. Now its 42px everywhere. Before this merge request certain row height varied from 40 to 45px. Sometimes even inside one list. 

cc @jschatz1 @connorshea 

See merge request !5666

Migrate protected branch access levels to match constants in `Gitlab::Access`

Closes #20606 

Context: https://gitlab.com/gitlab-org/gitlab-ce/issues/20606#note_13561628

See merge request !5658

Fix `#down` for two protected branches-related migrations.

- The migrations called `add_column_with_default` with a `null` option, which the Rails `add_column` method accepts. This fails because `add_column_with_default` expects an `allow_null` option instead.

- The migrations have been fixed to use `allow_null`.

See merge request !5660

This guide describes various commonly used operations and the impact
they have on the availability of a GitLab instance.

Refactor build artifacts documentation

## What does this MR do?

- Split user and admin documentation
- Use new location paths
- Add new Continuous Integration guide for the Admin area
- Link to new guide from the Admin area

## What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20612

See merge request !5662

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

Improve CI fixtures

cc @ayufan 

See merge request !5645

This will avoid lame conflicts when merging CE to EE
Signed-off-by: Rémy Coutable <remy@rymai.me>

Merge branch '20512-fix-rename-add-users-into-project-to-add-users-to-project-and-projects-ids-to-project-ids' into 'master'

Fix Rename `add_users_into_project` and `projects_ids`

## What does this MR do?
Only modifies the name of a method that leaves more semantic and expressive and the name of the keywords arguments to the rails convention.

## Are there points in the code the reviewer needs to double check?
Only if it has been changed at every point that is calling this method and that passing arguments.

## Why was this MR needed?
To make the code more expressive.

## What are the relevant issue numbers?
Closes #20512.

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [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 !5659

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Don’t close issues on original project

Closes #20527 -> https://gitlab.com/gitlab-org/gitlab-ce/issues/20527

See merge request !1981

Added to
* new_issue_email
* new_merge_request_email

Fix Import/Export not working in HA mode

Use a shared path instead of `Tempfile` default `/tmp` so the import file is accessible by any GitLab instance.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20506

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [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 !5618

We never add things `into` projects, we just add them `to` projects. So how about we rename this to `add_users_to_project`.

Rename `projects_ids` to `project_ids` by following the convention of rails.

Update docs for CoffeeScript -> JavaScript.

## What does this MR do?
Removes some CoffeeScript docs and adds more JS ones

## Are there points in the code the reviewer needs to double check?
No

## Why was this MR needed?
To migrate some docs

## What are the relevant issue numbers?
#20098

## Screenshots (if relevant)
None

## 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
  - [ ] Added for this feature/bug
  - [ ] 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 !5433

Merge branch '20491-remove-unnecessary-index_projects_on_builds_enabled-index-from-the-projects-table' into 'master'

Remove unnecessary index_projects_on_builds_enabled index from the projects table

See merge request !5611

Fix Devise deprecation warnings by updating omniauth provider paths to
use the new Devise/Rails 5 format.

See merge request !5331

Signed-off-by: Paco Guzman <pacoguzmanp@gmail.com>

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

Instrument Gitlab::Highlight

## What does this MR do?

This instruments the class `Gitlab::Highlight`.

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

No.

## Why was this MR needed?

This class is not instrumented.

## What are the relevant issue numbers?

#18592

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] 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)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5644

Fix confusing description of a blocked user.

## What does this MR do?
Rewrite confusing description of a blocked user in admin settings. If someone is blocked, the same message is displayed as if he is unblocked.

## Are there points in the code the reviewer needs to double check?
No.

## Why was this MR needed?
Because it seems that this is a cosmetic bug.

## What are the relevant issue numbers?
None.

## Screenshots (if relevant)
![blocked_user](/uploads/0782a82760ae19661d69c63a97daaf33/blocked_user.png)

## 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)
- [x] 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 !5582