MDL-21180 Gradebook: Consistent handling of change in aggregation

Change from/to GRADE_AGGREGATE_WEIGHTED_MEAN and GRADE_AGGREGATE_EXTRACREDIT_MEAN was only
handled prior to this patch. GRADE_AGGREGATE_SUM and GRADE_AGGREGATE_WEIGHTED_MEAN2 also
behave similar and should be handled same way
MDL-21180 Gradebook: Fixed coding style