Add command to remove empty Virtual Organizations

This MR adds a Django command that can be called from the command line and a method to the HelmholtzVirtualOrganization to remove empty VOs. You can either call it like

python manage.py remove_empty_vos

or from python via

from django_helmholtz_aai import models
models.HelmholtzVirtualOrganization.objects.remove_empty_vos()

ping @housam.dibeh @seyed.takyar

closes #1 (closed)