We had implemented our own login-required functionality inside the

@superuser_required decorator, but this didn't handle some of the API

changes that have happened upstream in Django. This change updates it so

that we just delegate to the built-in @login_required first.

Ran unit tests.
Tested that accessing various admin pages (DB, siteconfig forms) when

  logged out correctly redirected to the login form, and then back to

  the expected page.