Copy mail sender selection logic into MailReceivers

Summary: Copies sender identification logic into MailReceivers and makes it basically sane. The mess we run into after this try/catch is terrifying so I'm avoiding actually getting rid of any of it quite yet. Ref T1205.

Test Plan: Added a bit of test coverage. Used Receiver test console to verify some additional behaviors.

Reviewers: btrahan

Reviewed By: btrahan

CC: Afaque_Hussain, aran

Maniphest Tasks: T1205

Differential Revision: https://secure.phabricator.com/D5931