Add API key authentication to channels

[iiacoban42]

Use API key authentication before upgrading to websocket connection.

[StefanoSperetta]

This is a great example: https://reza-ta.medium.com/django-channels-web-socket-authentication-approaches-3a56954b4120

It provides 3 separate methods and, personally, I think method 2 (token/api-key in the authentication header) would be the simplest. If we use method 2, we might simply re-use most of the code used for the REST API authentication (even the authentication functions, so it is easier to test them) and leave the websocket completely transparent to authentication.