R1:c25595417f8a

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.