email: Message-Id header with user and thread ID

This patch suggests a change to the message-id creation process that includes stamping the receiving user-id (staff or client) along with the thread-id of the originating notice. This allows detection of threading if the clinically brain-dead mail client drops all the other header detection mechanisms, including the tag placed in the email body, on response.

This patch works for both client and agent communication.

*Hopefully this will make the next level of threading detection.*
... where the user's id and thread-entry-id are encoded in the message-id so the message can be threaded if it is replied to without any other indicator of the thread to which it belongs. This tag is signed with the secret-salt of the installation to guard against false positives