Allow repository policies to be edited

Summary: Ref T603. Allows permitted users to set view and edit policies for repositories. So far the repository list, repository detail, repository edit, and browse interfaces respect these settings. Most other interfaces will respect stricter settings, but "Public" won't work. Lots of rough edges in the integration still. None of this makes policies any looser than they were already without explicit user intervention, so I just put a warning about it in the UI.

Test Plan: Set a repository to public and browsed it. Verified I could not access non-public repositories.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, davidressman

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7061