WARNING cdn.mathjax.org end-of-life IN 5 DAYS

[Naereen]

Apparently, the main branch still uses the official MathJax CDN. WARNING It closes in 5 days! 💥 !

See https://www.mathjax.org/cdn-shutting-down/ for more details on migrating to another service.

Basically, a complete find-and-replace of https://cdn.mathjax.org/mathjax/2.7-latest/ (as is used right now) to https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/ could probably be enough.
But I prefer to let you chose which CDN to use instead of filing a pull request, hence this issue.

[hakimel]

Thanks for reporting! A fix (#1866) for this was already merged into the dev branch. I've added that to master now as well and it will be included in the 3.5.0 release.