[13.x] Payment page updates

[driesvints]

This PR refactors the payment page and will provide full support for the SEPA debit payment method as well as other payment methods. It now features a select dropdown when there's more than one payment method type available on the payment intent. The correct Stripe elements will be rendered for each one and if the payment method requires a redirect that'll also be handled properly. The PR also comes with support for saving payment methods (the ones that can do that) to the customer.

I didn't implement support for every single payment method but instead focused on the ones that seemed important enough:

• Credit Cards
• Alipay
• Bancontact
• BECS Direct Debit
• EPS
• Giropay
• iDEAL
• SEPA Direct Debit

Todo:

• Add payment method switcher depending on payment_method_types values
• Implement other payment methods
• Add checkbox to save payment method (Save Payment Method on payment page #1132)

Closes #435 #1132