MathJax issues loading fonts on macOS

[adhameer]

This is the same issue mentioned in a comment in #119:

We have also encountered some instances (on macOS: one on Safari 11.1.2, another on Chrome 69.0.3497.81, and some more on other macOS machines) where mathjax was attempting to load the STIX fonts that were removed from learn-ocaml's distribution, resulting in a [Math Processing Error]. There's probably some weird interaction with mathjax on macOS browsers.

Here's an error log:

document.getElementById("learnocaml-exo-tab-text").children[1].contentWindow.MathJax.Message.log
(13) [{…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}]
0: {text: "Typesetting math: 100%"}
1: {text: "Loading [MathJax]/jax/input/AsciiMath/config.js"}
2: {text: "Loading [MathJax]/jax/output/HTML-CSS/config.js"}
3: {text: "Loading [MathJax]/extensions/asciimath2jax.js"}
4: {text: "Loading [MathJax]/jax/element/mml/jax.js"}
5: {text: "Loading [MathJax]/jax/input/AsciiMath/jax.js"}
6: {text: "Loading [MathJax]/jax/output/HTML-CSS/jax.js"}
7: {text: "Loading [MathJax]/jax/output/HTML-CSS/fonts/STIX/fontdata.js"}
8: {text: "Loading [MathJax]/extensions/MathEvents.js"}
9: {text: "File failed to load: http://localhost/js/mathjax/jax/output/HTML-CSS/fonts/STIX/fontdata.js"}
10: {text: "Loading [MathJax]/jax/element/mml/optable/Latin1Supplement.js"}
11: {text: "Loading [MathJax]/jax/element/mml/optable/BasicLatin.js"}
12: {text: "Loading [MathJax]/jax/element/mml/optable/GreekAndCoptic.js"}
length: 13
__proto__: Array(0)