Implement modular transactions for application policy changes

Summary: Still needs some cleanup, but ready for review in broad outline form.

Test Plan:
Made lots of policy changes to the Badges application and confirmed expected rows in application_xactions, confirmed expected changes to phabricator.application-settings.

See example output (not quite working for custom policy objects) here:

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, chad, epriestley

Maniphest Tasks: T11476

Differential Revision: https://secure.phabricator.com/D17757