Mollie: Refunds/chargebacks handling
When the webhook detects a refund/chargeback on a payment
it will update the wallet balance and create transaction and payment record for
the operation. Receipts will display these (with a negative value).
Test Plan: ./phpunit
Reviewers: #hkccp_developers, vanmeeuwen
Reviewed By: #hkccp_developers, vanmeeuwen
Differential Revision: https://git.kolab.org/D1762