Give Phame blog posts configurable interact policies, with a default policy of "Same as Blog"
Summary: Ref T13661. This allows posts to have comments disabled (or restricted) on a per-post basis, and makes them inherit the containing blog policy by default.
Test Plan: Locked a post by editing its policy explicitly; locked a post by editing the containing blog policy.
Maniphest Tasks: T13661
Differential Revision: https://secure.phabricator.com/D21754