MathJax.js is not available in diffs since migration to jupyter_server

[krassowski]

Comparing jupyter_server/search?q=mathjax_url and notebook/search?q=mathjax_url it looks that:

• the handler was kept in the jupyter_server, but
• the actual mathjax javascript was removed from jupyter_server and is only present in notebook: notebook/static/base/js/mathjaxutils.js
• the mathjax config was also removed from jupyter_server and is only present in notebook: notebook/notebookapp.py#L1344-L1365

Originally posted by @krassowski in #566 (comment)

[krassowski]

One solution would be to default to the CDN, as is currently the case in jupyterlab_server:

https://github.com/afshin/jupyterlab_server/blob/0e5830a6e05492d1e82c1ae4ee7d12c8a5e074e9/jupyterlab_server/handlers.py#L92-L100

[vidartf]

I started work on https://github.com/jupyter-server/jupyter_server_mathjax to solve this.