Avoid serving the GA script to PageSpeed Insights

Google PageSpeed Insights tool often shows a "Leverage Browser Caching"
penalization when one is using Google Analytics with the recommended
installation method. This sounds very counter-intuitive given that both
tools come from Google themselves.

This works around that issue by conditionally loading Google Analytics
only if the page is not being crawled by Google PageSpeed Insights bot.

Google is cheating me, so I'm cheating Google back... ¯\_(ツ)_/¯

src/ssi/gtag.html

@@ -1,9 +1,24 @@
-<!-- Global site tag (gtag.js) - Google Analytics -->
-<script async src="https://www.googletagmanager.com/gtag/js?id=UA-108806299-1"></script>
 <script>
-    window.dataLayer = window.dataLayer || [];
-    function gtag() { dataLayer.push(arguments); }
-    gtag('js', new Date());
+    if (navigator.userAgent.indexOf('Google Page Speed Insights') === -1) {
+        var TRACKING_ID = 'UA-108806299-1';
 
-    gtag('config', 'UA-108806299-1');
+        var gtagElement = document.createElement('script');
+
+        gtagElement.async = true;
+        gtagElement.src = 'https://www.googletagmanager.com/gtag/js?id=' + TRACKING_ID;
+
+        var scriptElements = document.getElementsByTagName('script');
+        var lastScriptElement = scriptElements[scriptElements.length - 1];
+
+        lastScriptElement.parentNode.insertBefore(gtagElement, lastScriptElement.nextSibling);
+
+        window.dataLayer = window.dataLayer || [];
+
+        function gtag() {
+            dataLayer.push(arguments);
+        }
+
+        gtag('js', new Date());
+        gtag('config', TRACKING_ID);
+    }
 </script>