updated MathJax to v2.3

This updates MathJax to a slim copy of MathJax v2.3. The copy is trimmed down to only support MathML input, SVG output, replacing TeX fonts with STIX fonts (better unicode coverage); it also includes a few new MathJax extensions (contentMathML, matchwebfonts). See also  https://github.com/mathjax/MathJax-docs/wiki/Guide%3A-reducing-size-of-a-mathjax-installation