Transactions - make implementing TYPE_XXXX_POLICY transactions optional

Summary: Ref T6403. This was actually simple stuff.

Test Plan: changed the edit policy of a paste. changed the edit and join policy of a phame blog.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6403

Differential Revision: https://secure.phabricator.com/D12933