CLC-4321 - Added #export_options method to CanvasCourseGradeExportController, for use with front-end to validate export state and provide section selector.
Refactored #download_egrades_csv to use Canvas::Egrades#official_student_grades_csv for CSV download.
CLC-4321 - Migrated Course Grade Export authorization check request to factory
Added validation logic with errors in front-end.