Refactor playout endpoint

Replaced function json_playout with class APIPlayoutViewSet and added annotations.

Tested using latest aura-web deployment by replacing the steering image with its local Dockerfile:

  steering:
    container_name: steering
    build: 
      context: /home/fro/code/steering
      dockerfile: Dockerfile

Scheduled a show in dashboard and aura-playout retrieved the related data from the endpoint as expected - uploaded audio file started at desired time.