R1:d34b338f3f85

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:

{F4922240}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, chad, epriestley

Maniphest…