Update creation of placeholder user logic to work with placeholder user limits

When creating placeholder users, there are limits are based on plan and seats of the top-level namespace. When a top-level namespace reaches that limit, we should stop creating more placeholder users, and instead use the single import user for the top-level namespace (see #443553 (closed)).

The related issues #443553 (closed) will add the Import user, and #455903 (closed) will add the limit checking.

This issue is to update our code to no longer create placeholder holders once the limit has been reached, and instead to use the one import user created per namespace that was implemented in #443553 (closed), this would happen here.