Fixed some errors reported in google lighthouse audit tool.

src/template.html

@@ -4,6 +4,7 @@
   <meta charset="utf-8">
   <title><%= htmlWebpackPlugin.options.title %></title>
   <meta name="viewport" content="width=device-width, initial-scale=1">
+  <link rel="manifest" href="manifest.json">
   <style>
     html {
       background-color: rgb(28, 31, 36);