Support logged-out access to the document rendering endpoint

Summary: Ref T13105. Currently, logged-out users can't render documents via the endpoint even if they otherwise have access to the file.

Test Plan: Viewed a file as a logged-out user and re-rendered it via Ajax.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19258