R1:341079c3cf25

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…