This is the real fix to MDL-52573, but I wanted to do the simple fix there (and it wouldn't be backportable)

At the moment the singleview and user reports and hardcoded into the grader report. It should be possible to install other gradebook plugins and have them provide those row/column links from the gradebook.

Steps to reproduce:

• Develop a new fancy grading interface for a single user or grade item
• Install it to moodle

Expected results:

• Plugin should be abel to be linked to from grader report without having to hack core

This is sort of related to $CFG->grade_profilereport