Fix: legend sizing for MathJax names

[rajnisht7]

Bug fix:

When a trace's name contains LaTeX/MathJax (e.g. $\alpha_{1c} = 352$), the legend sometimes renders too small to fit the math cutting it off completely. This mostlyshows up in Firefox, but the underlying bug isn't Firefox-specific.
This PR tries to resolve this bug

Fixes #559

[emilykl]

Hi @rajnisht7, can you explain why this calculation gives the correct width and height?

[emilykl]

Thank you @rajnisht7 for the contribution! It would be great to fix this bug.

However I'm not totally confident that we can count on the viewBox attribute to get the correct width and height. If you think this approach is reliable, could you provide a bit more explanation?

If you can provide steps for reproducing and verifying the fix, that would also be helpful.

Also, we have recently updated the MathJax handling to support MathJax v4. Any approach used here will need to work with the new MathJax logic and support MathJax v3 and v4. Those changes are on the v4.0 branch, which will be merged into master on the release of the 4.0 version. So I would recommend that you either rebase these changes onto the v4.0 branch, or wait until v4.0 has been merged into master and verify whether the fix still works with the latest MathJax changes.

Thanks for your work so far!