Add hover color to emoji icon

## What does this MR do?
Adds hover color to emoji icon in discussions

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

## Why was this MR needed?
Keep consistency of color highlighting to indicate action icon

## What are the relevant issue numbers?
Closes #20629 

## Screenshots (if relevant)
Before:
![Gcy0V4Cjp9](/uploads/96a7a683fe2d526b74a05d7cc91c9fa0/Gcy0V4Cjp9.gif)
After:
![TAmusA6o1z](/uploads/1baf20390d2f1442b5fae84ae42914ce/TAmusA6o1z.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)

See merge request !5673