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 mail. Sent "!unsubscribe" mail.
Reviewers: btrahan, chad
Reviewed By: chad
CC: aran, tido
Maniphest Tasks: T3181
Differential Revision: https://secure.phabricator.com/D5911