Fixes #54

Make use of optimization trick for is_member

backend/backend_app/utils/__is_member.py

@@ -7,4 +7,7 @@ def is_member(group_name: str, user: User) -> bool:
 
     TODO unit test
     """
-    return user.groups.filter(name=group_name).exists()
+    return group_name in user.cached_groups
+    # before:
+    # When we were using the standard django model
+    # return user.groups.filter(name=group_name).exists()