Summary:
Depends on D19004. Ref T13053. Ref T12677. If the new `cluster.mailers` is configured, make use of it. Also use it in the Sengrid/Mailgun inbound stuff.
Also fix a bug where "Must Encrypt" mail to no recipients could fatal because no `$mail` was returned.
Test Plan: Processed some mail locally. The testing on this is still pretty flimsy, but I plan to solidify it in an upcoming change.
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T13053, T12677
Differential Revision: https://secure.phabricator.com/D19005
994d2e8e15