add field for default_sort_key

lms/lib/comment_client/user.py

@@ -9,6 +9,7 @@ class User(models.Model):
                          'id', 'external_id', 'subscribed_user_ids', 'children', 'course_id',
                          'subscribed_thread_ids', 'subscribed_commentable_ids',
                          'subscribed_course_ids', 'threads_count', 'comments_count',
+                         'default_sort_key',
                         ]
 
     updatable_fields = ['username', 'external_id', 'email']