Add an "Acting user" field to Herald

Summary:
Ref T13053. Fixes T7804. Adds "Acting user" so you can have "always email me" stuff skip things you did or keep an eye on suspicious interns.

For the test console, the current user is the acting user.

For pushes, the pusher is the acting user.

Test Plan: Wrote acting user rules, triggered them via test console and via multiple actors on real objects.

Maniphest Tasks: T13053, T7804

Differential Revision: https://secure.phabricator.com/D19031