Homestyx hydra

R1:1bf64e5cbcf4

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…
Repository: R1 hydra
Commit Date: Feb 6 2018