Merge pull request #270 from Genshin/class_groups

Class groups
Merge branch 'staging' of github.com:Genshin/GAKUEngine into staging
Add remove button for student selection
Fix eager loading in courses index
Fix eager loading in exams index
Fix eager loading in syllabuses index
Fix eager loading in teacher edit
Create student_selection based on redis
Add redis to travis
Totally WIP student_selection to class_groups
Merge branch 'student_selection' into staging
Remove for student_selection working and some cleaning
[fix] checking students with selection
Fix selected_students div in students index
Add sample data for applicant students
Merge branch 'master' of github.com:Genshin/GAKUEngine
Fix header
[factories] exam sessions and join between students
[models] exam sessions and associations
[migration] add exam sessions and time format preset