Use django-admin to precreate admin users

I looked at the playbook, and I see that we are using direct SQL access to switch people to superuser. I kinda wonder if it wouldn't be cleaner to use django-admin createsuperuser for that.

With the right config (as used in files/post-update.sh) and --noinput, the user can be created. Then we have to ask people to reset their password to connect, which would be secure enough (since there is email verification).