Closes Issue #25607

Added changelog.

Thanks to Greg Laubenstein for noticing in !6503

This reverts commit 434d98b2, reversing
changes made to b6a83be6.

This commit alters views for the following models to use the markdown cache if
present:

* AbuseReport
* Appearance
* ApplicationSetting
* BroadcastMessage
* Group
* Issue
* Label
* MergeRequest
* Milestone
* Project

At the same time, calls to `escape_once` have been moved into the `single_line`
Banzai pipeline, so they can't be missed out by accident and the work is done
at save, rather than render, time.

- Added controller actions as reusable concerns
- Added controller tests

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

* make sidebar button white
* make header button gray if not primary

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

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

With new navigation design it makes no sense to duplicate title in the
header

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

This ensures all these calls have an author set, allowing the use of
"all" mentions where possible.

This will also work better for when the title makes the buttons drop down a new line

Closes #14228

Currently this works by loading the HAML partials via XHR. While this is
not the nicest setup it _is_ the easiest setup using the tools we
currently have.

Loading this data asynchronously doesn't make loading the related
MRs/branches itself faster, it merely ensures that loading the issue
itself is not slowed down.

Fixes gitlab-org/gitlab-ce#14949

Closes #13878