Big update on the model based on our last meeting

Ressources:
- https://docs.djangoproject.com/en/dev/topics/db/examples/one_to_one/
- https://docs.djangoproject.com/en/dev/topics/db/examples/many_to_one/
- https://docs.djangoproject.com/en/dev/topics/db/examples/many_to_many/
- https://docs.djangoproject.com/en/dev/ref/models/fields/