Return the correct review update user from the last update resource

Review Request #10422 — Created Feb. 19, 2019 and submitted — Latest diff uploaded


Review Board


3d1a32c9d7 changed the last update resource to return the correct author of an
update when the change was published by a different user. Unfortunately this
made the author for review changes be the review request submitter and not the
review submitter.

Unit tests.

Also did a manual test where I created a review request with one user in window A, then posted a review with another in window B and returned to window A and verified that the updated bubble was correct (it wasn't before this change).