Add "Wait for feedback" button to change active participant in PR

In pull requests, it is difficult to know who is the active participant - the author or the reviewers.

The new ability of a reviewer to mark as "Needs work" greatly improves this, but it still isn't clear when the author has finished responding to comments and is ready for the reviewers to look again.

A "Wait for feedback" button or something similar would be good in order to pass over responsibility to the reviewer. Clicking this would send a notification to the reviewers immediately (before, no notifaction would be sent - unless some time has passed with no activity). Would be particularly useful when first annotating a review, or responding to a number of comments.