Add documentation for Review Apps

## What does this MR do?

Add docs for review apps.

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23484

See merge request !6982

Allows configuration of idle_timeout for incoming email.

https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1087

See merge request !7423

Improvements to Redis HA docs

Following https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6471

See merge request !7502

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Remove empty db fixtures file

## What does this MR do?
Removes empty fixtures file

See merge request !7519

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Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23484

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Refactor environments documentation

## What does this MR do?

Refactor environments docs.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23484

See merge request !7107

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Stop environment when branch is deleted

## What does this MR do?

This MR adds a environment teardown service, that is called when user deletes a branch. This most often happens when merge requests is merged.

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) 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

## What are the relevant issue numbers?

Closes #23218

See merge request !7355

Update CONTRIBUTING.md - Adding info about issues within a milestone

## What does this MR do?
Adds copy regarding issues within the current milestone to the contribution guide.

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) 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)

cc @dzaporozhets @awhildy

See merge request !7516

Revert "Merge branch '22680-unlabel-limit-autocomplete-to-selected-items' into 'master'"

There were some issues with the initial implementation: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6796#note_18557076

This reverts https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6796

See merge request !7521

Remove ToC and fix headings in Markdown docs

See merge request !7523

Adds hash anchors to diff_files to make possible linking exact file in commit

Closes #24010

See merge request !7298

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This reverts merge request !6796

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Change the slack notification comment link.

## What does this MR do?

Changes the link in Slack notifications for comments.

## What are the relevant issue numbers?

Closes #24107

See merge request !7498

Add a "gotcha" about FactoryGirl sequences to developer docs

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See merge request !7403

tests: Move rake assets:precompile and cloning gitlab-{shell,test} to the prepare stage

Related to #23034

See merge request !7512

Update the updated_at of a build while patching the trace

## What does this MR do?

Fixes build patching trace. It should update the `updated_at` field of a build to make sure it will not be removed by `stuck_ci_builds_worker`.

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

Construction of a test for the bug.

## Why was this MR needed?

Please read #22087 for a reference.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [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
  - [x] 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?

Fixes #22087

See merge request !7146

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Added colored labels to related MR list.

## What does this MR do?
Added colored labels to related MR list.

## Are there points in the code the reviewer needs to double check?
Yes, I think haml can be better.

## Why was this MR needed?
There was an issue for this #21200 

## Screenshots (if relevant)
![Screen_Shot_2016-11-15_at_21.04.24](/uploads/b601f33f23cdc3318d01279e17deb096/Screen_Shot_2016-11-15_at_21.04.24.png)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) 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
- [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?
Fixes !21200

See merge request !7486

explicitly disable eslint inline

## What does this MR do?

Explicitly disables eslint inline.

I.E. `/* eslint-disable */` => `/* eslint-disable max-len, no-new, ... */`

I used the script from https://gitlab.com/lbennett/eslint-dsblr.

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

## Why was this MR needed?

It will help refactor out eslint issues for older files.

## Screenshots (if relevant)

## 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
  - [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?

See merge request !7229