Respect role prices on admin order page

When an admin edits an order at admin/store/orders/%/edit, the form does not respect the role price of the customer.

Modify the form to use the correct role price of the customer.

The form for entering a new product there is uc_order_add_product_form.

Also, uc_price_per_role_find_price() will have to be updated to take the user as a parameter instead of just the current user. That way, it can use the customer roles instead of the current user roles.