middleware: Fix anonymous access to setup view

Django runs process_request() on all the middleware classes in order.
Then it runs process_view() on all of them again.  During the runs, if
any of the views return a HttpResponse.  Further processing is ignored.

Setup middleware was handling process_request() and returning
HttpResponse if the application as not setup.  However, stronghold
middleware handles process_view().  Since process_request happens first
and response is being returned there, stronghold middleware is never
called.

Make setup middleware use process_view() instead.

plinth/middleware.py

@@ -36,7 +36,7 @@ class SetupMiddleware(object):
     """Show setup page or progress if setup is neccessary or running."""
 
     @staticmethod
-    def process_request(request):
+    def process_view(request, view_func, view_args, view_kwargs):
         """Handle a request as Django middleware request handler."""
         # Perform a URL resolution. This is slightly inefficient as
         # Django will do this resolution again.