Fix Linkpoint gateway to sum the discount and subtotal

The Linkpoint gateway requires that the subtotal, tax and shipping equal the payment total. It completely ignores the discount provided, which could be the result of a promo code.

This branch adds the discount to the subtotal, and sets the discount to 0. While not strictly necessary to set to 0, I did not want to remove it completely since it breaks the contract already established that the gateway options include a discount key.