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.

Fix inconsistent highlighting of already selected activity nav-links

## What does this MR do?

*  Remove edge case where user could deselect an activity nav-link (which seems to be returning all the events)
*  Explicitly add an `All` tab to return all the events

## Are there points in the code the reviewer needs to double check?
Shouldn't be

## Why was this MR needed?
Resolves existing UI inconsistency

## Screenshots (if relevant)
Before:
![4OzkoQVJYc](/uploads/fd2a7fdbde2159e875482ec7b828fe60/4OzkoQVJYc.gif)

After:
![E0lj8UhEUU](/uploads/7eb5155861eb79d72957de04c9f172c9/E0lj8UhEUU.gif)

## Does this MR meet the acceptance criteria?

- [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)

## What are the relevant issue numbers?

*  Closes #21631
*  Closes #21452


See merge request !6091

Replace bootstrap caret with fontawesome caret

## What does this MR do?

1. Replaces all occurrences of `.caret` with fontawesome's caret (`fa-caret-down` for most cases)
2. Remove `.caret` from CSS
3. Refactored pipeline graph toggle to display the correct fontawesome caret (Had to change some JavaScript for this)  

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

*  Need to check if I missed any pages :disappointed_relieved: or for any side effects

## Why was this MR needed?
Resolves UI inconsistency

## Screenshots (if relevant)
Before (1 example out of many):
![Screen_Shot_2016-09-12_at_3.49.58_PM](/uploads/5eb93bbcc00412a5875e39302f4d8ba0/Screen_Shot_2016-09-12_at_3.49.58_PM.png)

After:
![Screen_Shot_2016-09-12_at_3.49.48_PM](/uploads/9259a9ebfb0bd09b4cfd55674e9b8be4/Screen_Shot_2016-09-12_at_3.49.48_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [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 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?
Closes #21169

See merge request !6314

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