Add support for PayPal in Adyen gateway

[remcotolsma]

Internal HelpScout ticket: https://secure.helpscout.net/conversation/1590353532/22475/

Test payment: https://woocommerce-adyen.wp-pay.org/wp-admin/post.php?post=3235&action=edit

https://docs.adyen.com/payment-methods/paypal/web-drop-in

[remcotolsma]

Re-informed in internal support ticket https://secure.helpscout.net/conversation/1624958858/22690?folderId=1425710.

[remcotolsma]

Some work-in-progress in pronamic/wp-pronamic-pay-adyen@97d8929 by @rvdsteege, but it's not working, i will have a look at this.

[rvdsteege]

Improvements by @remcotolsma in pronamic/wp-pronamic-pay-adyen@97d8929...d6b1098

I just fixed redirecting to payment status page after PayPal payment has been completed:

• pronamic/wp-pronamic-pay-adyen@c21fffc