refactor: add Google Tag Manager support

Author: mrholek

src/pug/_layout/default.pug

@@ -13,6 +13,10 @@ html(lang='en')
     block canonical
     block styles
   body
+    // Google Tag Manager (noscript)
+    noscript
+      iframe(src='https://www.googletagmanager.com/ns.html?id=GTM-KX4JH47' height='0' width='0' style='display:none;visibility:hidden')
+    // End Google Tag Manager (noscript)
     include ../_partials/sidebar.pug
     .wrapper.d-flex.flex-column.min-vh-100.bg-light
       include ../_partials/header.pug

src/pug/_partials/head.pug

@@ -32,4 +32,13 @@ link(rel='stylesheet' href='css/vendors/simplebar.css')
 link(href='css/style.css', rel='stylesheet')
 
 // We use those styles to show code examples, you should remove them in your application.
-link(href='css/examples.css', rel='stylesheet')
+link(href='css/examples.css', rel='stylesheet')
+
+// Google Tag Manager
+script.
+  (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
+  new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
+  j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
+  'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
+  })(window,document,'script','dataLayer','GTM-KX4JH47');
+// End Google Tag Manager