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, clicked it, got appropraite prompts.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3502