MDL-43856 MathJax: Dynamically load mathjax into the page (as required).

This is to solve a tricky edge case, when a page has no equations, and then an AJAX
action loads filtered content which contains an equation. (The glossary filter can do this).

To fix this I split the loading into configuration and typesetting. The config always happens,
but does not load the external mathjax libraries. The typesetting happens when an equation is
found - and is what triggers the load of MathJax. This can happen in response to an AJAX event.