Fix the login backend setting in the LTI Tool

Production settings include multiple authentication backends
(e.g., Shibboleth and Google social login).
The backend must be set in the `login()` function call.

lti_tool/views.py

@@ -69,7 +69,7 @@ def post(self, request, *args, **kwargs):
             profile.save()
 
             self.profile = profile
-        login(self.request, self.user)
+        login(self.request, self.user, backend='django.contrib.auth.backends.ModelBackend')
         self.request.session["lti-launch-id"] = message_launch.get_launch_id()
 
         if message_launch.is_resource_launch():