Add review state

Our users would like to be able to set a notification flag on the review saying that they are reviewing it (similar to crucible/fisheye) so that the requester and other reviewers know who's actively working on the code review.

This is imagined as:

• An additional button (or drop down from "approve") which has "start/stop progress" (similar to Jira)
• An additional symbol on reviewer images which shows who is reviewing it (similar to the green tick which is used to indicate that a review is approved by a user). Probably something like a yellow straight or looped arrow to indicate in progress.