Updated scripts for correctly using gtag. (alshedivat#2962)

Implementation of Google tag ID to correctly getting tracked requires
further modification in addition to the one line script in the
`_includes/scripts.liquid` file as
```
<script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics }}"></script>
  <script>
    window.dataLayer = window.dataLayer || [];
    function gtag() {
      dataLayer.push(arguments);
    }
    gtag('js', new Date());

    gtag('config', '{{ site.google_analytics }}');
  </script>
```

I have made the changes and tested it out on my personal webpage to
ensure the tracking in analytics dashboard.

Author: sr-dash

_includes/scripts.liquid

@@ -216,6 +216,15 @@
   <!-- Analytics -->
   <!-- Global site tag (gtag.js) - Google Analytics -->
   <script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics }}"></script>
+  <script>
+    window.dataLayer = window.dataLayer || [];
+    function gtag() {
+      dataLayer.push(arguments);
+    }
+    gtag('js', new Date());
+
+    gtag('config', '{{ site.google_analytics }}');
+  </script>
   <script defer src="{{ '/assets/js/google-analytics-setup.js' | relative_url | bust_file_cache }}"></script>
 {% endif %}