Proof site with html-proofer on change

Adds a script to the CI build to proof the Jekyll site if it has been changed in the last commit or PR, as suggested in #2601.

First checks with `git` to see if the `site/` directory has been changed. If it has, then continue, otherwise exit with code 0.

If `html-proofer` is not installed, install its nokogiri dependency with system libraries so it builds a lot faster. Then simply build the site with Jekyll and proof it.
Let's run this after all the tests! We could also scope the tests so they only run if a file was changed that would affect it.