Investigate upgrading mathjax in v2

[archmoj]

We can certainly upgrade to 2.7.9 and drop 2.3, especially if we can get this into our v2 release. In fact in a perfect world we'd upgrade to MathJax v3 in plotly.js v2, but I assume that'll be more than a one-afternoon change, so not something we can do right now.

Originally posted by @alexcjohnson in #5374 (comment)

[archmoj]

cc: #5395
@antoinerg could you please explore the possibility?

[nicolaskruchten]

@archmoj could you please explore this? @antoinerg is busy on other projects ATM :)