Only allow valid cart numbers
At the moment if I visit
https://keel.example.com/cart/991 I am directed to
https://shop.example.com/cart/991 even if cart 991 doesn't exist.
Please add a config option for a comma-separated list of "valid carts".
If the cart portion of the registration URL is missing or invalid, for example
https://keel.example.com/tier/991, it should resolve to the first number on the cart list. Ideally this should be in Keel itself (
https://keel.example.com/tier/1 ) but failing that, getting the correct shopping cart is fine (
https://keel.example.com/tier/991 -> SUBMIT ->