webwork2/html2xml doesn't use math4-overrides.css

[somiaj]

Embedded problems created using html2xml don't use math4-overrides.css, and any custom css is ignored. I noticed this in the problem library since it appears to now be using this to render the problems when viewed and also confirmed it happens in the few examples I've used embedded problems on other pages.

[drgrice1]

This is now done in ww 2.16.