Herald - unleash some recursive madness!

Summary: turn on herald rules ability to specify other herald rules. Fixes T4294.

Test Plan: made a rule to be cc'd on new tasks. made another rule to flag a task if it contained "test test" in the title AND the cc'd rule for new tasks matched. Made some new tasks and verified new "test test" tasks were flagged.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4294

Differential Revision: https://secure.phabricator.com/D8157