Put the first user to login in the default group

When using the AllowFirstUserDjangoBackend, the first user is allowed to log in
with any password they want. This change will make sure that the user is also
added to the default group, if one is configured.

desktop/core/src/desktop/auth/backend.py

@@ -157,6 +157,12 @@ class AllowFirstUserDjangoBackend(django.contrib.auth.backends.ModelBackend):
     if self.is_first_login_ever():
       user = find_or_create_user(username, password)
       user = rewrite_user(user)
+
+      default_group = get_default_user_group()
+      if default_group is not None:
+        user.groups.add(default_group)
+        user.save()
+
       return user
 
     return None