Use "cluster.mailers" if it is configured

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