[controller] change to find only students with active enrollment_status
[helper] select to use code instead of id
[model] enrollment status and student assoc, scope for find active enrollment statuses
[view] edit students select to use code instead of id
[view] search in students#index to use enrollment status code
[migration] change table for assoc with enrollment status
[spec/model] test enrollment_status_code attribute
[spec/req] fix syntax error
[spec/req] fix student edit enrollment status to click right option
[sample] fix creating students with enrollment status
Merge pull request #182 from Genshin/change_enrollment_status

Change enrollment status