Non-superusers can't sort users with alternate parent / template

[adrianbj]

Short description of the issue

I have admin users who have the required user-admin permissions, but they can't manually sort these users in the page tree.

Expected behavior

They should be able to sort them.

Actual behavior

They get the "You do not have permission to sort pages using this parent" error, even though they still see the "move" action button (which is something else that needs attention - it shouldn't appear if they can't sort).