Add management command to remove empty VOs

There should be a command to automatically remove empty VOs, something like

python manage.py autoremove_helmholtz_vos

Possible command-line options are:

• -e, --exclude: Exclude VOs that match the given pattern (ideally one can add this multiple times)
• -y, --yes: If set, do not ask for permission to remove a VO, rather just do it