refactor: Update Google Analytics code (#501)

layout/_partial/google-analytics.ejs

@@ -1,11 +1,10 @@
 <% if (theme.google_analytics_id) { %>
-    <script type="text/javascript">
-        (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
-        (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
-        m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
-        })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
+    <script async src="https://www.googletagmanager.com/gtag/js?id=<%= theme.google_analytics_id %>"></script>
+    <script>
+        window.dataLayer = window.dataLayer || [];
+        function gtag(){dataLayer.push(arguments);}
+        gtag('js', new Date());
 
-        ga('create', '<%= theme.google_analytics_id %>', 'auto');
-        ga('send', 'pageview');
+        gtag('config', '<%= theme.google_analytics_id %>');
     </script>
 <% } %>