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