Begin modularizing Herald fields
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.
Reviewed By: btrahan
Subscribers: eadler, joshuaspence, epriestley
Maniphest Tasks: T8726
Differential Revision: https://secure.phabricator.com/D13488