Pass coupon code in URL to auto add to cart

Allow the use of URL parameters to pass the coupon code in the order form URL so that we can automatically apply the coupon to the order. (still checking to ensure coupon code is valid for the products in the cart).

Clients are less likely to enter the coupon code when just given the code, if we can send an email with the promo and a link to order with the coupon code already activated it would give them less to figure out.