Add authentication via tokens

[TimJentzsch]

The current authentication method via email, username, password and API key is complicated to implement and not as easy to manage.

Instead, a simpler token authentication would be nice as an alternative.
This would also make it easier for third parties to use the API in the future.

Django REST provides a token-based authentication themselves, but it is quite limited (e.g. only one token per user).
They recommend the third-party package https://github.com/James1345/django-rest-knox which would probably be a good option for us.

[TimJentzsch]

For now we should just add this as alternative, not replacing the old authentication flow.
I think you also need to log in normally to obtain the token.