Prevent loops in received mail

Summary:
It's currently possible to configure Phabricator to send mail to some address it recognizes as relating to an object.

When we receive mail from Phabricator, drop it unconditionally.

Test Plan: Wrote two emails, one with the header and one without. Piped them to mail_handler.php, one was dropped immediately.

Reviewers: btrahan, nh, mikaaay, jungejason

Reviewed By: jungejason

CC: aran

Differential Revision: https://secure.phabricator.com/D2529