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