[assets] add select2 pluggin and make short function
[controller] fix typo in controller
[gem] select2
[view] add datepicker & select2 jquery pluggins
[spec/feature] simple grade enchance
[locale] simple grade type
Merge remote-tracking branch 'origin/fix_simple_grades' into implement_grading_methods

Conflicts:
	core/config/locales/en.yml