Google analytics update (#31)

* Update Google Analytics tracking code

Previous template was using ga.js which is considered legacy now. The
recommended way to enable Google Analytics now is via gtag.js:
https://support.google.com/analytics/answer/1008080

* Do not store IP address (personally identifyable information)

With GDPR in effect IP addresses are considered personally identifyable
information, and storing them becomes legally complicated. Anonymizing IPs
seems to be a sensible default behavior.

The only drawback is slightly reduced geolocation accuracy.

More information:
- https://developers.google.com/analytics/devguides/collection/gtagjs/ip-anonymization
- https://support.google.com/analytics/answer/2763052
- https://www.blastam.com/blog/5-actionable-steps-gdpr-compliance-google-analytics

templates/partials/analytics.js

@@ -1,13 +1,11 @@
 {% if GOOGLE_ANALYTICS %}
-    <script type="text/javascript">
-    var _gaq = _gaq || [];
-    _gaq.push(['_setAccount', '{{GOOGLE_ANALYTICS}}']);
-    _gaq.push(['_trackPageview']);
-    (function() {
-        var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
-        ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
-        var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
-    })();
+    <!-- Global Site Tag (gtag.js) - Google Analytics -->
+    <script async src="https://www.googletagmanager.com/gtag/js?id={{GOOGLE_ANALYTICS}}"></script>
+    <script>
+        window.dataLayer = window.dataLayer || [];
+        function gtag(){dataLayer.push(arguments);}
+        gtag('js', new Date());
+        gtag('config', '{{GOOGLE_ANALYTICS}}', { 'anonymize_ip': true });
     </script>
 {% endif %}
 {% if GAUGES %}