MathJax v2.7.1

This patch release to fix dependencies on cdn.mathjax.org which is nearing its end-of-life, cf. https://www.mathjax.org/cdn-shutting-down/ for migration help.

• Fix #1738
  • Add static copy of the MathJax Accessibility Extensions at v1.2.2.
  • Add static copy of mhchem at v3.2.0.
• Add latest.js, a convenience loader to load the latest release from cdnjs, rawgit and jsdelivr.