assets: Upgrade MathJax to 3.x and bundle it with webpack
A new major version of MathJax typesetter has been recently released which now enables to properly bundle the library with webpack.
So remove previous CDN use to get MathJax and use a self hosted version from now on.
Related to #1680 (closed)
Test Plan
There is already a cypress test to check math typesetting in notebook rendering.
Migrated from D3009 (view on Phabricator)