permission_required shouldn't cause redirect loop

[adehnert]

At the moment, AIUI (based on comments on zephyr) permission_required will check if somebody has the permission. If they don't, it will redirect to the login page. The login page will log them in (redundantly), and redirect to the page they came from. Naturally, they still don't have the permission, so they'll get redirected back to the login page. Repeat, until eventually the browser decides this is stupid.

Upstream has a raise_exception option. Unfortunately, what we really want to do is raise an exception if they're logged in, and otherwise redirect. We should make this happen, preferably by convincing upstream that this is the correct behavior so we don't need to use a custom permission_required.