The form page templates from the old application need to be merged into one. Each page can contain a form (to allow for synchronized input elements with the same name but on different form pages).
The form state must be synchronized across the forms to allow for easy page switching. Setting form values with jQuery val will not trigger any change events, see https://jsfiddle.net/ba33uwrt/5/
It must be possible to start the form process with a given global state and form page (Form values are given through POST). This'll allow to skip the payment page from banners.