Add widget for showing multiple certificates
    
    Certs widget is based on multivalued widget and adds ability to add new certificate
    and delete it. Each line is cert_widget.
    
    https://fedorahosted.org/freeipa/ticket/5108
    https://fedorahosted.org/freeipa/ticket/5381
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>