[controller] class group index action fetching class groups
[action/js] handling edit, delete classes that are in different semesters
[factory] class group trait with_semesters
[models] add dependent destroy, scope and class method for semester, class_group relations
[spec/model] testing dependent destroy for semester_class_groups assoc in clas_group model
[refactor/view] grouping class groups by semesters, and table for class_groups without semesters
[helper] extracting class_groups from semester_class_group relation
[spec/req] testing class group index refacotring by semesters
[gem] Add guard-cop
Merge branch 'master' into group_class_groups_index_by_semester

Conflicts:
	core/app/helpers/gaku/gaku_helper.rb