Modularize more Herald fields

Summary: Ref T8726. Continue making Herald fields more modular than they currently are.

Test Plan:

• Created a rule using all the affected fields.
• Ran the rule.
• Saw reasonable object field values.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: eadler, joshuaspence, epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13495