Added admin serializer for the event registrations

Gijs Hendriksen
2018-12-19T21:27:27+01:00

Added a separate admin serializer for the event registrations, in order to prevent data leaks. Also added an extra field to the events API to indicate whether a user has admin rights for a certain event.

### Steps to use this feature

1. Go to `/api/v1/events/[pk]/registrations` as normal user.
2. Notice the available information has been limited.
3. Go to `/api/v1/events/[pk]/registrations` as organiser of that event (or superuser).
4. Notice extra information is available.
5. Go to `/api/v1/events/[pk]/`.
6. Notice the `is_admin` field has been added.