Buy item an create user only after purchase

[georgmaisser]

• [x] Create anonymous user on add_item_to_cart functionality
• [ ] Make sure required files (custom profile fields) are set, so we don't run into redirect on checkout page.
• [ ] Ask for Name, Address etc. on successful payment (Retrieve it from Payment provider does not seem advisable)
• [ ] Change anonymous user on successful checkout and one time PW.