Remove approval of pull request when new commits are made after approval (BB-6784)

We really like the approval feature on Pull Requests, but we hit a situation today where I had approved something, then changes were made (new commits) but my approval remained, despite me having never seen the new changes.

We think it would be great if all approvals were reset when new commits are made to a pull request, otherwise we'll simply not use the feature.

Thanks!