Allow per contact encryption and digital signature

Outlook should allow auto-encryption and/or auto-signature on a per-contact
basis. This would allow one to indicate which of one's contacts are able to
accept and decrypt messages or are able to verify signatures. The current
implementation allows auto-encrypt and auto-sign on a global basis only.

This feature would allow users to slowly migrate to a secure email
environment one user at a time while avoiding the manual effort of selecting
the encrypt/sign option on each email sent.