R1:7b2b5cd91e01

Add basic support for a "Must Encrypt" mail flag which prevents unsecured content transmission

Summary:
Ref T13053. See PHI291. For particularly sensitive objects (like security issues), installs may reasonably wish to prevent details from being sent in plaintext over email.

This adds a "Must Encrypt" mail behavior, which discards mail content and all identifying details, replacing it with a link to the `/mail/` application. Users can follow the link to view the message over HTTPS.

The flag discards…