Make reply-to fully work in Maniphest and Differential for open source Phabricator

Summary:
Hook up the last pieces. This shouldn't impact the Facebook install, EXCEPT that
I removed "!accept" and added "!rethink" (plan changes). If you want to continue
supporting !accept, you should override the method in your subclass if you don't
already.

Test Plan:
Used the Mail Receiver test console to send mail to tasks and revisions.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran
Differential Revision: 289