Fix prepayment process in shopping cart

[georgmaisser]

Currently, there are the following elements on checkout.php prior to actually passing the payment over to the payment provider:

• Use credits
• Add Address
• Accept Terms
• Apply VAT

All these elements are added differently and work independently. The user flow is limited by this.

We should create one baseclass which handles the prebookingprocesses and makes sure all the transitions work in the same way.

• Develop the idea of what is shared between the components above and what is different
• Implement the framework
• Add use credits
• add address
• add accept terms
• add apply vat