Legalpad V0.2 - add mail integration

Summary:
Supports !unsubscribe and commenting on replies. Subscribers get mailed something reasonable. Fixes T3480.

Sneaks in /LX/ support. In the near future I want to have that /LX/ be a clean "signature" page sans all the edit actions and other fluff... Will resolve this as part of T3481.

Test Plan: used the metamta console to add comments and unsubscribe. added a phlog() inside mail code to verify mail bodies looked okay.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3480

Differential Revision: https://secure.phabricator.com/D6369