Support "public" policy in Tokens

Summary: Ref T4830. Allows logged-out users to browse tokens, policies permitting.

Test Plan: Browsed tokens as a logged-out user.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4830

Differential Revision: https://secure.phabricator.com/D8828