Generate a 404 page if Order is not found and cart is empty
Do not use magic method
Added method to interface IsotopeCheckoutStep

No BC break because it could never have worked without it
(so everyone already had this method implemented)
Added methods to interface IsotopePayment and IsotopeShipping

No BC break because it could never have worked without them
(so everyone already had these methods implemented)
Must be a Checkout module already because we depend on a lot of methods
CS
Fixed request token for integrity check