Add "Mute/Unmute" for subscribable objects

Summary: Ref T13053. See PHI126. Add an explicit "Mute" action to kill mail and notifications for a particular object.

Test Plan: Muted and umuted an object while interacting with it. Saw mail route appropriately.

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D19033