admin/user/create should redirect back to admin/user/create

Right now, when you go to add a user through the admin panel, it redirects you back to the list of users each time. This is tedious when you want to add multiple users, because you have to continue to click "add user" each time before filling out the form again.

This patch keeps you on the add form, and also brings the add user form more inline with other administration pages such as adding taxonomy terms, etc.