Change default MathJax URL to v3 rather than v2

[emilykl]

Closes#462

The next major Plotly.js release will drop MathJax v2 support, so before releasing that version in Plotly.py we will need to update Kaleido to use MathJax v3 (or v4) rather than v2 by default, so that everything works smoothly together.

I've made the change to v3 in this PR, but v4 would also be reasonable IMO.

[camdecoster]

Could you provide some info about the sections of code that have been removed? Were those only required for MathJax v2?

[emilykl]

@camdecoster Sure. These two are definitely no longer needed:

• '?config=TeX-AMS-MML_SVG' was the old way of specifying a specific configuration when loading MathJax; starting with v3, you just link directly to the specific *.js file that you want (tex-svg.min.js in our case).
• window.PlotlyConfig = {MathJaxConfig: 'local'} was an old pattern that plotly.js used for configuring the way the library uses MathJax (see an example here), but MathJaxConfig is no longer referenced in the codebase

On the other hand, MathJax.Hub.Configno longer exists starting with MathJax v3, butthere's a new API for setting config options and this line should probably be ported over to the new API. I'll make that change.

[emilykl]

@camdecoster Update on the config options: I added the snippet

<script>MathJax={svg: {blacker: 0}};</script>

to the page template, which would be the MathJax v3 equivalent of the v2 config I removed, but I see zero visible difference in the resulting image, so I'm inclined to just leave it out.