Update MathJax code to match LMS JS code

In MDL-79039 the MathJax JS code in LMS was rewritten from YUI to AMD. It works the same as before, but some naming has changed (e.g. it no longer uses this.window.M.filter_mathjaxloader) so it could be nice to update the app code to reflect the new naming.

Also, in MDL-79042 a new event was added to notify when the Mathjax equations have finished rendering. This event doesn't seem to be used yet, maybe it's meant for plugins or it will be used in the future. So we might want to add a similar event in the app.