Enable analytics

[broadwell]

Enables Google Analytics on the site, through the Jekyll hooks that were mostly already in place.

NOTE that the environment variable JEKYLL_ENV=production must be set when building the site to deploy it to production in order for analytics to enabled. This value must not be set on any other build; otherwise our local, staging, etc. sites will all send reports to Google as well.

[simonwiles]

I've added some additional GA config -- these comments are on the commit:

• adding ga("set", "transport", "beacon"); saves HTTP requests and improves performance.
• adding ga("set", "anonymizeIp", true); anonymizes IP addresses.

  By default Google Analytics records the full IP address of a user visiting the site, but that full IP address is never available to the Google Analytics property admin. By anonymizing the IP address you can make your site more GDPR-compliant as a full IP address can be defined as PII (personally identifiable information.)