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