Fix commit policy stuff and anchor handling

Summary:
See discussion in D5121. Fixes T2615.

This might cause us more issues if anything is loading commit handles without passing a viewer, but I think I tested all of those cases.

Test Plan: Looked at feed, audit, maniphest, diffusion, differential, owners, repositories.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T2615

Differential Revision: https://secure.phabricator.com/D5139