MDL-25544 MDL-46658 gradereport_grader: Update JS to meet coding style guidelines

This also moves the code into a set of separate files to allow for
additional components of the report to be migrated to the same YUI module,
adjusts some UI components, improves JS performance, and adds appropriate
documentation.
MDL-25544 MDL-46774 gradereport_grader: Move row/column highlight code to YUI module
MDL-25544 MDL-46731 gradereport_grader: Move tooltips to YUI module
MDL-25544 MDL-46776 assignment: Add generation of assignments to the course generator
MDL-25544 MDL-46663 gradereport_grader: Remove old scrollview code
MDL-25544 MDL-46663 gradereport_grader: Remove fixed column support