[mathjax2] Set charset attribute explicitly
[mathjax2] Set charset attribute explicitly

In my browser, the entire url is serialized, *including*
the charset= line. This makes a URL like
https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-AMS_CHTML-full,Safe&amp;delayStartupUntil=configured%22%20charset=%22utf-8%22,
which fails to load.

jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab