R1:6b1c27eb0ead

Make "public" pastes meaningfully visible to logged-out users

Summary:
- Introduce `shouldAllowPublic()`, indicating that logged-out users are OK in a controller if the install is configured to permit public policies.
- Make Paste views and lists allow public users.
- Make UI do sensible things with respect to disabling links, etc.
- Improve behavior of "you need to login" with respect to policy exceptions and Ajax requests.

Test Plan: Looked at "public" paste, saw all unavailable UI disabled…