Tasks PaymentScreen

[grunerlisa]

• [x] Change Navigation from "Ändern" Button to last screen instead of FindBikeNests
• [x] Scrap Promo Code
• [x] Ebike ignored in Backend
• [x] Change Zahlungsmethode to IBAN and Häkchen for SEPA Lastschriftmandat
• [x] Show Zahlungsmethode only when not already set ("Go to your profile to change your Zahlungsinformationen")
• [x] Check IBAN for Length (in Frontend) -> only possible to reservate when IBAN is accepted
• [ ] Check IBAN for Prüfziffer (in Frontend) -> only possible to reservate when IBAN is accepted