and add a new manager for active members.

- Put admin views in a separate file
- Move creation/updating/cancelling of registrations to services.py
- Create classes for all frontend views so that these are class-based
- Make small changes to registration behaviour and urls (update form now loads using get)
- Make small changes to model methods: change some into properties or remove them in total and add new ones for better global usage
- Add API for registrations

Use event instead of event pk for

Add is_registration_allowed

Do not use status for is_registration_allowed

Rename is_registration_allowed, add cancellation_allowed and tests

Completely remove event 'status'

Catch DoesNotExist exceptions

Fix tests and PEP8

Rename is_registration_allowed to _is_registration_allowed

Move event attendance check to member model

Remove status property from events

Fix API fields

Add deprecated status field to events serializer

Fix event_id to pk

Fix serializers and viewsets

Change errors of update_registration

Make  a default empty hidden field

Fix fields in admin

Return latest data after update

Add tests for API

Fix all_present

Fix services create registration return

Previously `Member`s with multiple `Membership`s were listed

Makes sure the organiser is always set.

Places a minimum of 5 (euros) on the fine field, as is policy by the current board.

* Models and admin page for events
* Add organisers to events
* event overview page for organisers