PayPal donations

**WIP**
It looks like URL of the IPN listener is set in account settings in the current version. Should I keep it like this and remove `notify_url` parameter or pass it to PayPal?

There's more info about this in their docs at https://developer.paypal.com/docs/classic/ipn/integration-guide/IPNSetup/.

> If you enable the IPN service, PayPal sends messages to the IPN listener at the URL you specify in your account profile. If you want, you can override this URL in order to associate a different IPN listener with a specific transaction. To do this, you can either:
> 
> * Specify the URL of a different listener in your definition of a PayPal Payment Standard button 
> or
> * Pass the URL of a different listener to a call of a PayPal API operation