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