Additional fields manual registration cannot be altered

Ghost User
2019-04-03T14:09:56+02:00

When non-members do a registration for an event, we cannot fill in the additional fields (like Allergies for the Introduction Barbecue) manually and because they have no account they cannot do it either.

### Desired behaviour
In the list of registrations or on the page for editing a registration it would be nice to be able to fill in those additional fields.