Support image diffs in Rapid Diffs

What does this MR do and why?

Implements #499294 (closed).

Screenshots or screen recordings

Before	After	Current MR diffs

How to set up and validate locally

1. Enable rapid_diffs and rapid_diffs_on_mr_show feature flags
2. Go to (or create) a merge request with image diffs
3. Select 'Changes' tab
4. Add ?rapid_diffs=true to the URL, follow it
5. Observe image diffs displaying correctly