R1:d0c648dfa53b

Make "Can Interact" and logged-out users interact more gracefully

Summary:
Fixes T12378. Two minor issues here:

- CAN_INTERACT on tasks uses "USER", but should just use the view policy, which may be more permissive ("PUBLIC").
- CAN_INTERACT is currently prevented from being "PUBLIC" by additional safeguards. Define an explicit capability object for the permission which returns `true` from `shouldAllowPublicPolicySetting()`.

Test Plan:
- Viewed an unlocked task as a logged-out user, saw "login to…