analytics: Upgrade to gtag.js to enable GA4

docs/shared/tpl/googleanalytics.html

@@ -1,14 +1,15 @@
 [%- IF siteconfig.googleanalytics %]
 [%# https://developers.google.com/analytics/devguides/collection/upgrade/reference/gajs-analyticsjs %]
-<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=[% siteconfig.googleanalytics %]"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
 
-  ga('create', '[% siteconfig.googleanalytics %]', 'auto');
-  ga('require', 'linkid', 'linkid.js');
-  ga('set', 'dimension1', window.location.protocol);
-  ga('send', 'pageview');
+  gtag('config', '[% siteconfig.googleanalytics %]', {
+   'link_attribution': true,
+   'custom_map': {'dimension1': 'protocol'}
+  });
+  gtag('event', 'pageload', {'protocol': window.location.protocol});
 </script>
 [% END -%]