Fixed slash commands not loading

## What does this MR do?

Fixes an issue with slash commands not working when the autocomplete source is loading & then the new issue button is clicked. This also fixes an issue where the autocomplete source is loaded on pages where it isn't actually needed.

## What are the relevant issue numbers?

Closes #21774, #21807

See merge request !6207

Frontend for Merge Request Diff

This merge request improves the UX for the merge request diff feature which was recently implemented here (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6127). Specifically, it styles various parts of the diff feature to match the designs, it disables comment-related buttons in states where comments are disabled, and it adds a 'Show latest version' button for convenience.

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

I could use feedback on this MR's fidelity to the design.  

## Why was this MR needed?

Neccessary styling improvements for basic UX of this feature, and enabled comment buttons are not functional and thus need to be disabled in certain states.

## Screenshots (if relevant)

![57dd0755f0b14342305909](/uploads/318a44a3bc8b7fc5c9c6ef92ba92e511/57dd0755f0b14342305909.gif)

## 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
  - [ ] 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 you do - 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/21427
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6127

cc: @jschatz1 

See merge request !6343

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

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

* Created a force=true param that will continue with the previous
  behaviour of the unsubscribe method
* Created a filter for not-logged users so they see a unsubsribe
  confirmation page
* Added the List-Unsubscribe header on emails so the email client can
  display it on top

* upstream/master: (80 commits)
  Contributing via GH no longer encouraged.
  Update README.md to really fix icon
  Update README.md to fix icon
  Allow to set request_access_enabled for groups and projects using API
  Restrict last_activity_at updates to one per hour
  Solve code review comments
  Properly support Gitlab::Auth::Result
  Revert all changes introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6043
  Move Gitlab::Auth.Result to separate file
  Added CHANGELOG
  Fix spec failures
  Support pushing via SSH
  Fix specs failures
  Don't leak build tokens in build logs
  Fix permissions for creating container images
  Fix old CHANGELOG entries
  Add linting for duplicate CHANGELOG versions (!6039)
  Ensure validation messages are shown within the milestone form
  Fix validation regexs (+1 squashed commit) Squashed commits: [f9a9315] Use : to test invalid environment name
  Fix scope of the CI config key nodes in jobs entry
  ...

Import/Export security specs

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20857

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/20821/

See merge request !1987