existing mathjax link is outdated

[hrbigelow]

In chromium 71 at least, MathJax doesn't render due to an outdated link. I used:

for f in $(find . -name '*.html');
do cp $f $f.orig;
perl -lne '$_ =~ s%(?<!<!--)(<script .+? src=")(.+?)(?config=TeX-AMS-MML_HTMLorMML")%${1}https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js$3%; print $_' $f.orig > $f;
rm $f.orig;
done

to update the links, as recommended in https://www.mathjax.org/cdn-shutting-down/

I spot-checked pages in chromium and firefox, and the equations render correctly.