Migrate from analytics.js to gtag.js

We should move this to the internal Hugo template when tested.

themes/gohugoioTheme/layouts/_default/baseof.html

@@ -34,7 +34,7 @@
       {{- template "_internal/twitter_cards.html" . -}}
 
       {{ if eq (getenv "HUGO_ENV") "production" | or (eq .Site.Params.env "production")  }}
-        {{ template "_internal/google_analytics_async.html" . }}
+        {{ partial "gtag" . }}
       {{ end }}
 
 

themes/gohugoioTheme/layouts/partials/gtag.html

@@ -0,0 +1,10 @@
+{{ with .Site.GoogleAnalytics }}
+<script async src="https://www.googletagmanager.com/gtag/js?id={{ . }}"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', '{{ . }}');
+</script>
+{{ end }}