Avoid CDN use for math

[gpaumier]

The default behavior of nikola is to load MathJax from a CDN.

Better: load a local copy and/or include it in the theme's assets, depending on size

Best: Use KaTeX and pre-render the math text during the nikola compilation process, so that only the CSS and fonts need to be loaded by the reader's browser, but not the JS (see pelican-katex for inspiration)

[gpaumier]

Even better: there's a nikola plugin for local KaTeX pre-rendering, so use that (and self-hosted assets).