MDL-47078 Grades: Allow sum of grades and exclude empty grades to be friends.

This change modifies the sum_of_grades thing to store the calculated max grade in the
rawgrademax for the grade_grade for the category. It then changes the reports to use the
rawgrademax / rawgrademin for displaying grade values instead of using the grademin/grademax
from the grade_item.
MDL-47078 Grades: Remove warning from form about excludeempty and sum of grades.