MDL-10386 Refactored the grader report file (index.php), and extracted the grade_repo...

MDL-10386 Refactored the grader report file (index.php), and extracted the grade_report_grader class out of it. The index file now only calls upon the class' main methods. Certain elements and code blocks in the class can be abstracted for other reports (such as the paging, sorting arrows and several general purpose variables).