Convert image diff background image to CSS
## What does this MR do?
Replaces the `trans_bg.gif` image used for image diff background image with a CSS implementation of the same background image
## Are there points in the code the reviewer needs to double check?
None
## Why was this MR needed?
Uses CSS to replace the use of an image so that webpage rendering is slightly improved
## What are the relevant issue numbers?
Closes#20500
## Screenshots (if relevant)
Before:
After:
## 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 !5598
3552900733
