Split dummy generator

[rolandgeider]

trafficstars

At the moment the dummy data generator is a single script in extras/dummy_generator. It would be cleaner to move the individual functionality to a management command in each app. Since some apps have different dummy data (e.g. nutritional plans and nutrition diary entries) we should decide how to handle this, either in separate commands python3 manage.py dummy-nutrition-plan 10 or python3 manage.py dummy-nutrition plans 10. Also, since the docs mention this, they should be edited as well