Make Phame blog policies non-nullable

Summary: Ref T6203. Ref T13661. These policies are incorrectly nullable, although it's likely that no pathway exists in the application to write NULL into them. Fix the schema.

Test Plan: Ran bin/storage upgrade.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13661, T6203

Differential Revision: https://secure.phabricator.com/D21748