R1:994d2e8e1563

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…