Fix commit mention font inconsistency

## What does this MR do?
Removes the mono font styling for the auto linked commit hashes.

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

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

## What are the relevant issue numbers?
Closes #20936 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-15_at_1.48.13_PM](/uploads/9f8728e91442b087f723e73edfe8893e/Screen_Shot_2016-08-15_at_1.48.13_PM.png)

After:
 ![Screen_Shot_2016-08-15_at_1.48.26_PM](/uploads/d48d877337f9f3673cdffd434ff368cb/Screen_Shot_2016-08-15_at_1.48.26_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 [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)

See merge request !5817