Add payment reason on payment email notifications

[mruestes]

When we're setting up a new email notification after a payment, it would be great to know the reason for the payment (renewal, new customer, upgrade, etc). In this way, we could send more specific messages when we thank a customer for their payment, i.e. Thank you for your renewal.

This information is on the amendments that generated the invoice and the payment, but it's not available when setting up a notification.