Fix pizza order permissions

Ghost User
2019-08-28T19:10:52+02:00

Closes #872

### Current behaviour / Reproducing the bug

1. Make a pizza event and add some orders
2. Your user should not be a superuser
3. Try to mark the orders as paid in the frontend
4. Clicking doesn't do anything

### Expected behaviour

1. Make a pizza event and add some orders
2. Your user should not be a superuser
3. Try to mark the orders as paid in the frontend
4. The orders page is permission denied