Add CartInterface::queuedUpsertItem and CartInterface::route

[pdbreen]

Cart item queuing is required to support email login after an "add to cart" button is clicked. It does this by making the add-to-cart POST routes accessible w/o requiring login. The controller for the post creates the cart item and then calls queuedUpsertItem(...) which will either add the item immediately, or defer addition by storing in session data until a user login even occurs. By redirecting to the cart (which is an auth protected route) after addition, the login page will appear when needed. And, because the cart is a GET and not a POST, laravel's redirect to intended will work properly, showing the cart after email capture.

[drewroberts]

Thank you! 🚏