Add google-analytics

[gbaz]

It would be very good to have some objective data on browsing patterns, to see how people tend to use the site. This would really inform future discussions.

I just registered us a key under haskell.org -- we just need to add it to all the pages.

Code follows (to be inserted in HEAD section):

<!-- 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>

[gbaz]

resolved