Conpherence - make messages to btrahan@metamta.domain start conpherences

Summary: I'm not super happy with the prettiness of the code, but I wasn't able to come up with a good way to clean it up. Happy for suggestions.

Test Plan: sent message to btrahan@phabricator.dev from gmail. Copied raw email and piped it to mail_handler.php -- it created a conpherence! Repeated but sent to btrahan and xerxes and noted that the conpherence was created for both users

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2431

Differential Revision: https://secure.phabricator.com/D4854