Add "X-Auto-Response-Suppress" header to all outgoing mail

Summary: This appears to sometimes be effective (for MS clients), and we've seen it in the wild on inbound mail.

Test Plan: Sent myself some mail, verified it had the right header.

Reviewers: btrahan, vrana, jungejason

Reviewed By: vrana

CC: aran

Maniphest Tasks: T571

Differential Revision: https://secure.phabricator.com/D2241