Add an explicit "this mail came from Phabricator" header

Summary:
See T926. If you want to write a mail rule that, e.g., captures Differential
mail but ignores people replying to it, it's kind of tricky right now. You can
use the 'X-Mail-Transport-Agent' header but that's not obvious and it's not
necessarily stable.

Add a nice, obvious "X-Phabricator-Sent-This-Message" header.

Test Plan: Sent myself some mail, verified the header appeared.

Reviewers: vrana, btrahan, fugalh, jungejason

Reviewed By: jungejason

CC: aran, epriestley

Maniphest Tasks: T926

Differential Revision: https://secure.phabricator.com/D1732