do_create_user: Use transaction.atomic.

Have kept process_new_human_user out of
the atomic block because it involves many
different operations and also sends events.
Tried enclosing event in on_commit but that
would need many changes in the tests, so have
skipped it for now.

zerver/lib/actions.py

@@ -655,7 +655,7 @@ def do_create_user(
     acting_user: Optional[UserProfile],
     enable_marketing_emails: bool = True,
 ) -> UserProfile:
-
+    with transaction.atomic():
         user_profile = create_user(
             email=email,
             password=password,