WebUI: Add Custom command multivalued adder dialog
    
    Adder dialog which is used along with custom_command_multivalued_widget.
    It behaivor of confirm dialog and adds fields which are necessary.
    
    Part of: https://fedorahosted.org/freeipa/ticket/6601
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>