R1:ded641ae3270

Add basic per-object privacy policies

Summary:
Provides a basic start for access policies. Objects expose various capabilities, like CAN_VIEW, CAN_EDIT, etc., and set a policy for each capability. We currently implement three policies, PUBLIC (anyone, including logged-out), USERS (any logged-in) and NOONE (nobody). There's also a way to provide automatic capability grants (e.g., the owner of an object can always see it, even if some capability is set to "NOONE"), but I'm not sure how great the…