Unapprove PR automatically when a PR has been changed

As a reviewer, when an author updates a PR and adds new commits to their branch, I want the review status to change from approved to unapproved automatically so that I can quickly find out which PR needs my attention.

It would also be useful to be able to identify which file has changed, so I don't have to go thru all full commit. I know I can look under the commits tab, but it still takes a while to figure out where to begin with the new additions.

(Note: These basic features available in Crucible.)