Use Differential policy columns to drive policies

Summary:
Ref T603. Read policies out of policy columns.

When a revision is associated with a repository (which is currently never), require view access on the repository to see the revision (or, require the viewer to be the owner). This is a blanket "do the right thing" rule which should make Differential's default policies align with user expectations.

Future diffs will populate the `repositoryPHID` when a revision is created.

Test Plan: Tooled around Differential. None of this stuff does anything yet, so nothing very exciting happened.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7134