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 comment" instead of "locked".
• Viewed a locked task as a logged-out user, saw "locked".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12378

Differential Revision: https://secure.phabricator.com/D17485