Hovercards - allow public access

Summary: see title

Test Plan: set config to allow public access and viewed a hovercard uri. saw a hovercard with little info as opposed to login prompt. Fixes T6337.

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: Korvin, epriestley

Maniphest Tasks: T6337

Differential Revision: https://secure.phabricator.com/D10722