Changes

For simplicity, and since Differential's review system does not map cleanly to BMO's review flags, r+ flags, and only r+ flags, will be set on the stub attachment associated with a Differential revision when a Phabricator user performs an "Accept Revision" action. The flag will be removed if the user later issues a "Request Changes" or a "Resign as Reviewer" action. Similarly, all r+ flags will be removed if the author selects any of the "Plan Changes", "Request Review", or "Abandon Revision" actions. In the last case, the stub attachment will also be obsoleted.