Add support for configurable analytics

[froggleston]

This PR aims to implement configurable analytics to address #438 with a new config.yaml option, analytics.

Options:

• Default option for analytics is NULL, or not supplying it, and therefore disables tracking for that lesson
• analytics: carpentries will add the Carpentries matomo tracking script to the footer.html (see corresponding varnish PR)
• analytics: | <user_string> will add the user supplied tracker script string, e.g. for google analytics:

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

    gtag('config', '{YOUR TRACKING ID}');
  </script>

[froggleston]

Thanks @tobyhodges ! I agree. The addition of user-specified tracking code would hopefully be fairly transparent, as tracking code would be committed alongside any lesson. I think that at least having the option to include or exclude Matomo tracking would make this PR worth it.