Allow XHPAST to be viewed public

Summary: Ref T4830. Not sure this should be public (consume resources?)

Test Plan: View logged out.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4830

Differential Revision: https://secure.phabricator.com/D13674