Fix registration problems
This MR fixes two things in the registrations app.
Payment not attached to Registration
Previous behaviour
Steps to reproduce:
- Create a new registration
- Payment not added to registration after accepting
New behaviour
Steps to validate that it works:
- Create a new registration
- Payment added to registration after accepting
Fields not shown in admin
Previous behaviour
Steps to reproduce:
- Create a new registration
- Missing fields in the admin
New behaviour
Steps to validate that it works:
- Create a new registration
- Missing fields in the admin