user_topics: Make bulk_set_user_topic_..._in_database atomic.

In e726012244, we removed the
@transaction.atomic decorator from the function
'bulk_set_user_topic_visibility_policy_in_database'.

We should keep it to avoid adding it to the caller
of this function, when reused in the future.

zerver/lib/user_topics.py

@@ -126,6 +126,7 @@ def get_topic_visibility_policy(
     return visibility_policy
 
 
+@transaction.atomic(savepoint=False)
 def bulk_set_user_topic_visibility_policy_in_database(
     user_profiles: List[UserProfile],
     stream_id: int,