Load demo database not via sql

[tpokorra]

we have issues with sequences with postgresql vs sqlite.

better: have our own manage function, and load python objects, or use django dump and load data fixtures (json).

• https://docs.djangoproject.com/en/4.0/howto/custom-management-commands/
• https://docs.djangoproject.com/en/4.0/ref/django-admin/#loaddata