Herald - add support for task priority

Summary: adds a new FIELD and a new VALUE to support this. Slightly dodgy because priorities do not have phids so we have to special case how we handle this in a few spots. Ref T4294.

Test Plan: made a new rule to get cc'd on unbreak now and wishlist tasks. verified got cc'd correctly and not cc'd correctly.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4294

Differential Revision: https://secure.phabricator.com/D8156