Add MathJax to markdown rendering of clarifications

[eldering]

At the WFs at Baku we saw a team write their clarification request with inline math, e.g. $n$. It would be nice to support this and more advanced math with MathJax.

[vmcj]

My suggestion would be to use: https://packagist.org/packages/mathjax/mathjax

We would need to think about some situations which are valid in MathJax & Markdown and make sure they don't break.

[vmcj]

@eldering would we also be fine with MathML?
#3113

The advantage of MathML is that we can handle everything on the server.

[eldering]

@eldering would we also be fine with MathML? #3113

The advantage of MathML is that we can handle everything on the server.

See my comment there. I don't think that's realistic alternative to MathJax.