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?

What are the relevant issue numbers?

Closes #2228.