Add the ability to mark individual files as Reviewed or Needs Work

I know it's been a while since this suggestion was first raised (in BSERV-4274) and I'm not sure why it's marked as resolved because the functionality doesn't exist but I think this would still (4 years later) be a great addition to the PR workflow.

As a reviewer, I don't have the ability to mark an individual file to say, "Yes, I've reviewed the changes to this file and I'm happy with them to go to our Production environment". I know that when a change to the file is committed it is shown in bold styling in the PR diff file tree but this disappears after clicking it once and then if you come back to the pull request at a later date, you're not sure whether you finished reviewing and approved that file or not.

If a pull request has 50 file changes and I have been reviewing them one by one I'd like to check off the ones I've completed reviewing. For example, if a particular file has a one line change in it and it hasn't been changed again since I first reviewed it, I'd like to forget about that file as I know it has been approved. On the flip side, if a new commit was pushed which contained changes to a previously 'Approved' file, the green 'tick' next to the file in the diff tree would be automatically removed prompting me to review that individual file again.