Make Differential views capability-sensitive

Summary:
Ref T603. Make Differential behaviors for logged-out and underprivleged users more similar to other apps.

I'm going to drop this "anonymous access" thing at some point, but `reviews.fb.net` actually looks like it's running semi-modern code, so leave it alive until we have a more compelling replacement in the upstream.

Test Plan: As a logged out user, browsed Differential and clicked things and such.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7148