Support Herald rules for Herald rules

Summary:
Depends on D19399. Ref T13130. This adds basic support for writing Herald rules against Herald rules. See T13130 for a lot more detail.

This needs a bit more work to be useful: for example, there's no way to specify the rule type or subject, so you can't say "notify me when global rules are edited" or "notify me when Maniphest rules are edited". I'll add some fields for that in followup changes to actually solve the original use case.

Test Plan:

• Wrote Herald rules against Herald rules.
• Ran them by editing rules and in the test console.
• Verified they sent some mail with bin/mail list-outbound.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13130

Differential Revision: https://secure.phabricator.com/D19400