Shopping Cart and Product Infor Quantity

The "Must Agree To Terms" contribution enhances standard OSC Checkout logic by forcing the customer to
agree to the store's terms in order to place a new order.

Requirements:
- Add the quantity of the available products on a drop-down menu of the inventory quantity. In this way, the customer could choose the correct value on the range between 1 and the quantity available;
- The information must be up-to-dated to the state of the stock, so has to be checked with the database and with the cart to synchronize the quantities;
- Both drop-down menus displayed on the product_info.php and shopping_cart.php pages must show the same information about the number of items on the shopping cart.