Use Chosen for currency select menu
We don't use $_SESSION['CHECKOUT_DATA'] anymore.

The data is stored in the order if appropriate (payment_data)
CS
Hide shipping address if it is the same as billing address in checkout review
Do not hide checkout next button on error

It could be a temporary error like a missing input field. If the step
really fails (e.g. no payment method available), it is a shop
misconfiguration and not a case the user should ever see...
Log an error message if payment or shipping is not available (misconfig)