[view] fix to use counter_cache
[factory] add traints and fixes
[migrate] add coutners to enrollmentable and student
[controller] use enrollmentable count
[migrate] add comment for course count attribute
[spec/model] test counters
[model] refactor enrollment assoc for readability
[model] handle course, class_group and extracurricular activity counts in student record
Merge branch 'enrollmentable' of github.com:Genshin/gaku into enrollmentable