Modularize Herald task status/priority fields

Summary: Ref T8726. These are a bit involved because they have custom rendering and editor values.

Test Plan: Created new rule using these fields, edited tasks to trigger them, viewed transcripts.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: eadler, joshuaspence, epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13500