Fix more js errors

public/js/extra.js

@@ -1,5 +1,6 @@
 var hljs = require('highlight.js');
 var PDFObject = require('pdfobject');
+var S = require('string');
 
 //auto update last change
 var createtime = null;

webpack.config.js

@@ -15,7 +15,8 @@ module.exports = {
             emojify: "emojify.js",
             io: "socket.io-client",
             LZString: "lz-string",
-            key: "keymaster"
+            key: "keymaster",
+            Viz: "viz.js"
         }),
         new webpack.DefinePlugin({
             "require.specified": "require.resolve"