Notify approvers when their approvals are invalidated

First, thanks for BCLOUD-7042

Now, when the branch of a pull-requests gets updated, all approvals get removed.
Now, as author of the pull-request, I have to look at the history to ping the people who approved before this change to re-approve the latest changes.

Our suggestion to fix this, is to add the people who approve a PR to the list of reviewers. This way, after a change comes in, it's easy to keep track of who needs to re-approve.

EDIT: original request included commenters. After talking this over again, the main problem is when someone gives an approval on a PR, a new commit comes in, we have to look in the history of who approved the original PR to ask for re-approval.

It would be really nice if people who give their approval on a PR would be added to the list of 'reviewers' so we know who to poke for a re-approve after changing the PR

Staff edit:
Updating title to reflect how we would probably solve this (by notifying approvers instead of modifying the reviewers list).