Properly bundle MathJax once version 3.0 is released

Incoming Jupyter notebook rendering feature for swh-web/browse (!96 (closed)) uses MathJax under the hood to perform math typesetting.

The current version of MathJax (2.7.5) does not play well with a module bundler tool like webpack [1] so the current behavior is to load the library from a CDN.

Hopefully, the next major version of MathJax will bring proper support for webpack processing [2]. So, we should be able to drop the CDN use once version 3.0 gets released.

[1] https://github.com/mathjax/MathJax/issues/1629
[2] https://www.mathjax.org/MathJax-v3-beta.3/

Migrated from T1680 (view on Phabricator)

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