Add Editor-based mail stamps: actor, via, silent, encrypted, new, mention, self-actor, self-mention

Summary: Ref T13053. Adds more mail tags with information available on the Editor object.

Test Plan: Banged around in Maniphest, viewed the resulting mail, all the stamps seemed to align with reality.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D18995