Set up environment variables

Adding environment variables will allow us to differentiate between local development and the live, published site. This is great for things like ensuring our analytics are accurate, as well as better CI/CD integration (not to mention clever things like creating favicons to indicate if the site is live or not when working on it).

Here's a good starting point on setting this up in Eleventy: https://weblog.herok.pl/posts/eleventy-tip-use-environment-variables/