Implement group purging

For sites upgrading to CKAN 2.0, migrating a site's existing groups to organizations seems a likely common request. You can't create an org with the same name as an existing group, even if the group is deleted. So we need a way to either migrate a group to an org in-place, or purge a group from the db so a new org with the same name can be created. Purging seemed more generally useful.