Implement EventID to scope ClientIDs and Entry IDs

Implement an EventID saved in settings. Currently this is used to scope
clientIDs and entryIDs to an event. The client checks the event currently going on on
the server, and discards its localstorage (containing the clientID) if
it has changed

backend/templates/songlist.html

@@ -79,7 +79,7 @@
         $.ajax({
             type: 'POST',
             url: '/api/enqueue',
-            data: JSON.stringify(data), // or JSON.stringify ({name: 'jonas'}),
+            data: JSON.stringify(data),
             success: success_callback,
             statusCode: {
                 423: blocked_callback
@@ -99,6 +99,7 @@
         enqueue(localStorage.getItem("clientId"),id, name, function () {
             $("#enqueueModal").modal('hide');
             window.location.href = '/#end';
+            
         }, function (response) {
             bootbox.alert({
                 message: "Deine Eintragung konnte leider nicht vorgenommen werden.\nGrund: "+response.responseJSON.status,