Mask the sender for "Must Encrypt" mail

Summary:
Depends on D18998. Ref T13053. When we send "Must Encrypt" mail, we currently send it with a normal "From" address.

This discloses a little information about the object (for example, if the Director of Silly Walks is interacting with a "must encrypt" object, the vulnerability is probably related to Silly Walks), so anonymize who is interacting with the object.

Test Plan: Processed some mail. (The actual final "From" is ephemeral and a little tricky to examine and I didn't actually transmit mail over the network, but it should be obvious if this works or not on secure.)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D19000