Publish books with the most-open policy

Summary: Ref T4558. When publishing a new Diviner book, use the most-open policy instead of PhabricatorPolicies::POLICY_USER.

Test Plan: Ran diviner generate in a directory which didn't have published Diviner documentation.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4558

Differential Revision: https://secure.phabricator.com/D13254