Add django extension to generate the class diagram of the models

This merge is a proper version of !157 (merged).
Extension and the tool to generate the picture comes from debian apt.
I also want to keep trace of the command used to generate the class diagram of the models, and maybe the resulting picture.
For the moment I have it in my django/docs as :

#! /bin/bash

# It should be ran in the django container.
# cf https://django-extensions.readthedocs.io/en/latest/graph_models.html
python3 ./manage.py graph_models --all-applications --group-models --output docs/allgo_models.png

I don't know what is its final place.