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.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13661

Differential Revision: https://secure.phabricator.com/D21754