webui: hide empty fields and sections
    
    Hide widgets without a value. Must be explicitly turned on. In widget by
    `hidden_if_empty` flag. Or globally by `hide_empty_widgets` flag. Global
    hiding can be individually turned off by `ignore_empty_hiding` flag.
    
    https://fedorahosted.org/freeipa/ticket/4402
    
    Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>