CLC-4321 - Added Canvas::Egrades model to provide data related to eGrades export
CLC-4321 - Added #official_sections method to CanvasCourseGradeExportController, for use with front-end to choose appropriate export section
CLC-4321 - Converted CanvasCourseGradeExportController#official_sections into #export_options, now serving grade types available, as well as sections terms present
CLC-4321 - Migrated Course Grade Export authorization check request to factory
CLC-4321 - Added #official_student_grades_csv to Canvas::Egrades
CLC-4321 - Finished #download_egrades_csv in CanvasCourseGradeExportController
CLC-4321 - Added logic to AngularJS front-end for Canvas Course Grade Export for handling export state validation