Make X-Herald-Rules header sticky

Summary:
This isn't terribly elegant but it solves the problem without loss of
generality. We can pursue a more finessed solution later if it seems prudent.

Test Plan:
Created a revision matched by a blanket herald rule, and then commented on it.
Comment email had X-Herald-Rules header in it.

Reviewed By: aran
Reviewers: aran, tuomaspelkonen, jungejason
CC: aran
Differential Revision: 218