Export of events registrations is broken

One-sentence description

Current behaviour

HTTP 500

Traceback:  

File "/usr/local/lib/python3.5/site-packages/django/core/handlers/exception.py" in inner
  41.             response = get_response(request)

File "/usr/local/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
  187.                 response = self.process_exception_by_middleware(e, request)

File "/usr/local/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
  185.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/usr/local/lib/python3.5/site-packages/django/contrib/auth/decorators.py" in _wrapped_view
  23.                 return view_func(request, *args, **kwargs)

File "/usr/local/lib/python3.5/site-packages/django/contrib/auth/decorators.py" in _wrapped_view
  23.                 return view_func(request, *args, **kwargs)

File "./events/views.py" in export
  123.             data['paid'] = _('Yes') if registration.paid else ''

Exception Type: AttributeError at /events/admin/212/export/
Exception Value: 'Registration' object has no attribute 'paid'

Expected behaviour

No errors

Steps to reproduce

1. Create an event
2. Add registrations
3. Export all registrations

Possible solution

Fix the export to use the new payment field instead of paid.