Have Return-URL after donation is complete

[Lineflyer]

We can customize PayPal to link back to an URL after a donation is completed.
It could be used to show a "Thank you!" page e.g. also explaining for what and what not donations are used.

There are only some minor requirements we have to meet for Paypal:

- Per the user agreement, you must provide verbiage on the page displayed by the Return URL that will help the buyer understand that the payment has been made and that the transaction has been completed.

- You must provide verbiage on the page displayed by the Return URL that explains that payment transaction details will be emailed to the buyer.

- Example: Thank you for your payment. Your transaction has been completed, and a receipt for your purchase has been emailed to you. You may log into your account at www.paypal.com/de to view details of this transaction.