"Pay by card" button

[Madis Väin]

Support adding a button to pay the invoice with a click via Stripe, Pin Payments, etc.

• https://support.stripe.com/questions/send-customers-to-stripe-com-to-make-a-payment
• https://pinpayments.com/small-business-payments/invoices

[Steffen Geipel]

@madisvain maybe we should think about to add PayPal as well?