Give Phame blogs mutable interact policies

Summary:
Ref T13661.

I'm fairly sure these policies don't actually do anything (you can't "interact" with a blog) but the primarily support a Phame Post object policy of "Same as Parent Blog", which is the "natural" interact policy for a post.

Most of this is infrastructure support for mutable interact policies: today, only Maniphest has interact mutability and only via indirect effects (locking tasks), not through a directly mutable "Can Interact" policy.

Test Plan:
Ran storage upgrade, edited interact policy of a blog, saw appropriate persistence and transactions.

Created and edited a task to make sure there's no weird fallout from increasing what can be done with interact policies.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13661

Differential Revision: https://secure.phabricator.com/D21751