Embed google analytics script

[mitchellchou]

Ref

https://3.basecamp.com/3096882/buckets/25656427/messages/5156230749#__recording_5177684368

<!-- Global site tag (gtag.js) - Google Analytics -->
<script async src="https://www.googletagmanager.com/gtag/js?id=G-5ZZTRBCR0N"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'G-5ZZTRBCR0N');
</script>

[mitchellchou]

only production