htmlutils: render MathML by MathJax

• Adds config variable CFG_MATHJAX_RENDERS_MATHML to enable or disable rendering of MathML via MathJax.
• Changes function responsible for loading MathJax to load different configuration version according to CFG_MATHJAX_RENDERS_MATHML.
• Amends part numbering in invenio.conf.

Signed-off-by: Wojciech Ziolek <wojciech.ziolek@cern.ch>
Reviewed-by: Samuele Kaplun <samuele.kaplun@cern.ch>