Track "accepted" and "commented" in per-reviewer status

Summary: Ref T1279. Updates status to 'accepted' or 'commented' when the user takes those actions.

Test Plan:

• Commented on a revision, got a comment icon.
• Accepted a revision, got an accept icon.
• Commented again, icon stayed as "accept".
• Faked the "old diff" states.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7229