(captive portal) fix multiple voucher generation, closes...

(captive portal) fix multiple voucher generation, closes https://github.com/opnsense/core/issues/728

(cherry picked from commit 85630abc)

src/opnsense/mvc/app/views/OPNsense/CaptivePortal/vouchers.volt

@@ -195,6 +195,10 @@ POSSIBILITY OF SUCH DAMAGE.
                         });
 
                         // generate download link and send data to the client
+                        if ($('#downloadFile').length) {
+                            // remove previous link
+                            $('#downloadFile').remove();
+                        }
                         $('<a></a>')
                                 .attr('id','downloadFile')
                                 .attr('href','data:text/csv;charset=utf8,' + encodeURIComponent(output_data))