add GA tag

resolves #665

Author: gbaz

datafiles/templates/hackageCssTheme.st

@@ -1,3 +1,13 @@
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <link rel="stylesheet" href="/static/hackage.css" type="text/css" />
 <link rel="search" type="application/opensearchdescription+xml" title="Hackage" href="/packages/opensearch.xml" />
+<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=UA-83290513-3"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', 'UA-83290513-3');
+</script>
+