Add Differential and Herald mail stamps and some refinements

Summary:
Ref T13053. Adds revision stamps (status, reviewers, etc). Adds Herald rule stamps, like the existing X-Herald-Rules header.

Removes the "self" stamps, since you can just write a rule against whatever(@epriestley) equivalently. If there's routing logic around this, it can live in the routing layer. This avoids tons of self-actor, self-mention, self-reviewer, self-blocking-reviewer, self-resigned-reviewer, etc., stamps.

Use natcasesort() instead of sort() so that numeric values (like monograms) sort 9, 80, 700 instead of 700, 80, 9.

Remove the commas from rendering since they don't really add anything.

Test Plan: Edited tasks and revisions, looked at mail stamps, saw stamps that looked pretty reasonable (with no more self stuff, no more commas, sorting numbers, and Herald stamps).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D18997