Cache highlighted content for diff note files
Currently we do cache the note raw diff content for discussions on
note_diff_files table but we don't cache the highlighted content. I believe there's room to greatly improve the loading time of the MR discussions by doing so.
Profiling a MR with lots of diff comments:
We currently have a separate class for diff highlighting cache Gitlab::Diff::HighlightCache. I believe we could extend that to not only cache the highlighting of a whole diff collection, but for separate diff files as well.
Downside: Spend more memory caching a reasonably big content.