Use "cluster.mailers" if it is configured


Use "cluster.mailers" if it is configured

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