added server.js and fixed "npm start" script for heroku deployment

package.json

@@ -4,7 +4,7 @@
   "private": true,
   "description": "HTML Admin Template",
   "scripts": {
-    "start": "webpack-dev-server",
+    "start": "npm run build && node server.js",
     "dev": "webpack-dashboard -t 'Project' -- webpack-dev-server",
     "clean": "shx rm -rf ./build",
     "build": "npm run clean && cross-env NODE_ENV=\"production\" webpack",

server.js

@@ -0,0 +1,12 @@
+const express = require('express');
+
+const app = express();
+
+app.use(express.static('public'));
+
+// start the server with the build folder.
+app.use('/', express.static(__dirname + '/build'));
+
+const server = app.listen(process.env.PORT || 3000, function () {
+  console.log('Server started at http://localhost:%s', server.address().port);
+});