Implement the deferred payment flow

[DragoonAethis]

The scenario:

• You pay for the ticket, everything works, yay
• P24 sends a notification to our gateway endpoint that it worked, our handler verifies/accepts the payment and marks it as confirmed.
• P24 redirects the user back to Coriolis, handler checks if the payment is confirmed and updates the ticket status accordingly.

In most cases, the event order is just that, but occasionally something goes wrong and the notification shows up late.

The problem is that it's the user's view handler doing the ticket status updates - if P24 notification shows up after the redirect, it never gets updated. While this can be fixed by clicking Pay Online again (and django-payments will just do the redirect from the 3rd step), it's not a great experience.

• An easy fix is to update ticket statuses in the gateway endpoint handler, but if the payment fails, nothing ever hits it. Not great.
• The 3rd step should redirect to a "waiting for payment" screen where we poll for ticket status updates for a few seconds.
• If that doesn't happen, a background task that asks P24 what's up should fire.