WIP: Improve new/edit MR approver settings UX

What does this MR do?

This improves the new/edit MR page to have the latest UX for the approver selection.

This was originally closing #1526 (closed), but I now see that is actually completely out of scope. Please ignore the branch name.

Now closes #2228.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2228.