Closes no issue
Short description
This MR moves the admin urls of payments to the backend, it adds fields to the Payment model to keep track of who paid for and who processed a payment and the payment widget has the process buttons integrated now (see screenshot), then I added a notes field that can be used to write down what a payment is/was for (if we start removing registrations/renewals for the GDPR this may be worth a lot) and wrote a migration that fills in the paid by/notes fields for the existing payments created for registrations. Last but not least: I moved the processed state into the type field. Basically because those fields could either be both filled or both empty, now the payment is seen as processed when the type field is not equal to no_payment which is now the default value instead of None.
This last change massively improves the UX of the registrations package, since going to the payment, processing and going back is now one action: click the process button.
