Add a Herald action to trigger "Must Encrypt" for mail

Summary: Depends on D18983. Ref T13053. Adds a new Herald action to activate the "must encrypt" flag and drop mail content.

Test Plan:

• Created a new Herald rule:

• Created a "dog task" (woof woof, unsecure) and a "duck task" (quack quack, secure).
• Viewed mail for both in bin/mail and web UI, saw appropriate security/encryption behavior.
• Viewed "Must Encrypt" in "Headers" tab for the duck mail, saw why the mail was encrypted (link to Herald rule).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D18984