MDL-43856: Improvements to the MathJax filter

* Remove all traces of mathjax from the atto equation editor - it is filter agnostic
* Add a M.core.event module for core events that can be published/subscribed to by plugins (loose coupling)
* Add a FILTER_CONTENT_UPDATED event so plugins can tell the filter system when nodes are added to the page
* Implement a listener for this event in the MathJax filter
* Fire this event from the equation editor and from the glossary filter
* Add a nolink tag to the mathjax filter, so equations will not be processed by the other filters
* Add a smarter throttle function to the equation editor preview updates (Y.throttle is dumb)