Prevent shopping cart form submission for completed orders

If you open the shopping cart form in multiple tabs and complete checkout in one of them, submitting the open shopping cart form from another tab will reset the order status of the completed order to "Checkout: Checkout." The shopping cart form does not have equivalent checks to the checkout router's that prevent access to checkout pages for an order after the order has progressed beyond a break point in the checkout process (loosely defined as a page with no back form button).

We should add an alteration to the shopping cart form that checks the order status during the form build process, since that's the earliest we can get to it, to prevent shopping cart form submission when the shopping cart order ID no longer matches a user's current cart order ID or when the order status indicates the order has progressed beyond a break point. To prevent submission of the shopping cart form in these cases, we'll use a redirect to whatever the current page is.