Move some received mail responsibility to applications

Summary:
Ref T1205. Continuation of D5915.

Currently, `PhabricatorMetaMTAReceivedMail` has //all// the logic for routing mail. In particular:

  - New mail receivers in applications must edit it.
  - Mail receivers don't drop out when applications are uninstalled.

Applications have some logic in subclasses of `PhabricatorMailReplyHandler`, but this class is a bit of a mess. It is also heavily based on the assumption that mail receivers are objects (like revisions), but this is not true in at least two cases today (creating new tasks with `bugs@`, creating a new Conpherence thread) and likely other cases in the future (e.g., revision-by-mail).

Move this logic into a new `PhabricatorMailReceiver` classtree. This is similar to `PhabricatorMailReplyHandler` but a bit cleaner and more general. I plan to heavily reduce the responsibilities of `PhabricatorMailReplyHandler` or possibly eliminate it entirely.

For now, the new classtree doesn't do much of interest. The only behavioral change this diff causes is that Phabricator will now reject mail to an application when that application is uninstalled.

I also moved all the `ReplyHandler` classes into `mail/` directories in their respective applications.

Test Plan: Unit tests, used receive test to route mail to various objects.

Reviewers: btrahan

Reviewed By: btrahan

CC: Afaque_Hussain, edward, aran

Maniphest Tasks: T1205

Differential Revision: https://secure.phabricator.com/D5922