R1:9e6da4220611

Fix some issues with Maniphest inbound email

Summary:
Fixes T3181.

- Inbound `bugs@` mail is broken right now if it doesn't use the new external user stuff, because it calls `$user->getPhabricatorUser()` on an object which is already a `PhabricatorUser`. Instead, build the right `$user` object from the external user earlier on.
- Maniphest mail is nuking or otherwise awkwardly altering CCs. Make this work properly.
- Make sure "!unsubscribe" works correctly.

Test Plan: Sent `bugs@` mail. Sent CC…