Make execution order of Herald rules explicit

Summary: Fixes T6211. This gives Herald rules an explicit execution order, which seems generally good. See some discussion on T6211 and inline.

Test Plan:
  - Added unit test.
  - Dry ran rules and saw rules appear in the expected order in the transcript.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6211

Differential Revision: https://secure.phabricator.com/D10624