Add Expand and Collapse all files buttons to merge requests

What does this MR do and why?

Implements Collapse / Expand all diffs (#361278 - closed).

This merge request adds expand and collapse all files buttons to merge requests. These buttons can also be triggered with hotkeys: ; and shift+; respectively.

Screenshots or screen recordings

Screen_Recording_2024-09-06_at_06.08.21

How to set up and validate locally

1. Open any merge request with changes
2. Expand all file and collapse all files buttons should be available next to merge request preferences dropdown
3. Click these buttons
4. Observe all files either collapse or expand