This implements the "better" approach outlined in this doc.
It posts to slack when someone submits a review or leaves a top-level comment
If someone leaves a review (without approve/request-changes), and that person was also the last person to leave a review, then the second notification gets suppressed. This is a way to cut down noise when people reply to comments, or do a review without the bulk "start a review" functionality. (In both cases, each individual comment becomes a comment-only review of its own; this would spam the slack thread)
This implements the "better" approach outlined in this doc.