Implement Kassa Compleet PaymentService

[skamoen]

ING offers "Kassa Compleet", which is an alternative payment service. It's cheaper per transaction and easier for the treasurer to manage.

Documentation can be found here:
https://s3-eu-west-1.amazonaws.com/wl1-apidocs/api.kassacompleet.nl/index.html

Seems easy enough to implement, though it will need some adjustments as the webhook and redirect URL are the same for this API.

[praseodym]

ING Kassa Compleet has a fixed cost of €9,95/month but is cheaper by €0,04/transaction compared to Mollie, so you'd need 248 transactions/month for it to be cheaper.