Review and move AJAX grading code to YUI module

We have an AJAX interface for updating grades in the gradebook, but it's off by default and it's also pretty buggy.

johno has started looking at it, the issues currently surrounding it, and is trying establish whether we want to enable this by default and move it into the YUI module.

This is currently exploratory work and we'll need to discuss whether this is something we want enabled by default, or whether it will lead to overridden grades too frequently.

https://docs.moodle.org/dev/Grader_report_2014#Main_display