Upgrade MathJax to v3

Once #12 is in, it may be worth upgrading MathJax to v3

See https://docs.mathjax.org/en/latest/upgrading/v2.html for a guide
and https://mathjax.github.io/MathJax-demos-web/convert-configuration/convert-configuration.html for the configuration converter

Also, vendor the library for CSP compliance