Merge branch 'fixed-width-caret' into 'master'

Add width to caret to rid ourselves of the shift

## What does this MR do?
Removes the shift from diff files.

## Are there points in the code the reviewer needs to double check?
That it doesn't shift.

## Why was this MR needed?
There was a shift in the minimizing of diff files.

## Screenshots (if relevant)
![switcheroo](/uploads/f9734e1d11715e49008e0ca9104d2d4c/switcheroo.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
  - [ ] 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?
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23424

See merge request !6934

CHANGELOG.md

@@ -6,6 +6,7 @@ entry.
 
 - Show correct environment log in admin/logs (@duk3luk3 !7191)
 - Fix Milestone dropdown not stay selected for `Upcoming` and `No Milestone` option !7117
+- Diff collapse won't shift when collapsing.
 - Backups do not fail anymore when using tar on annex and custom_hooks only. !5814
 - Adds user project membership expired event to clarify why user was removed (Callum Dryden)
 - Trim leading and trailing whitespace on project_path (Linus Thiel)

app/views/projects/diffs/_file_header.html.haml

-%i.fa.diff-toggle-caret
+%i.fa.diff-toggle-caret.fa-fw
 - if defined?(blob) && blob && diff_file.submodule?
   %span
     = icon('archive fw')