Convert switch icon into icon font

## What does this MR do?
Converts the switch icon into the exchange icon from font awesome

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

## Why was this MR needed?

*  Decreases an external resource for the repository compare page
*  Icon font provides a sharper image for displays with higher pixel resolutions

## What are the relevant issue numbers?
Closes #20508 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-01_at_3.06.06_PM](/uploads/6ad46cc6dff98ec534e1f5a0fe556fa7/Screen_Shot_2016-08-01_at_3.06.06_PM.png)

After:
![Screen_Shot_2016-08-01_at_3.06.10_PM](/uploads/3cb11c40cbba398f9986a4cc6b3a7e4c/Screen_Shot_2016-08-01_at_3.06.10_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [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 !5603