Fix review request counters when reassigning ownership.
Review Request #417 - Created Feb. 20, 2017 and submitted
Reassigning review request ownership worked pretty well overall, but would leave the review request counters untouched. This change fixes that, and adds a unit test to verify the results. Testing done: - Manually reassigned a review request and saw the pending/total counters update correctly. - Ran unit tests. Reviewed at https://reviews.reviewboard.org/r/8757/