Fix excessively-conservative feed story policy checks

Summary:
When checking if a user can see a feed story about an object, we currently use the object's primary policy but ignore automatic capabilities.

Instead, proxy both primary policies and automatic capabilities.

Test Plan:

• Before this patch, users could not see stories about events they were invited to but not permitted to see by the primary policy (this is currently the default for newly created events).
• After this patch, these invited users can now see the stories.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: lpriestley, epriestley

Differential Revision: https://secure.phabricator.com/D12785