Begin modularizing Herald fields

Summary:
Ref T8726. This lays groundwork for modularizing Herald fields and modularizes the "Content source" field.

This doesn't touch conditions or values yet.

Test Plan: Created a rule with a "content source" field, created a new task, saw rule apply.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: eadler, joshuaspence, epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13488