R1:696a1b22babf

Make feed stories properly respect object policies

Summary:
- When a feed story's primary object is a Policy object, use its visibility policy to control story visibility. Leave an exception for
- Augment PhabricatorPolicyAwareQuery so queries may do pre-policy filtering without the need to handle their own buffering/cursor code. (We could slightly improve this: if a query returns less than a page of pre-filtered results we could keep getting pre-filtered results until we had at least a page's worth…