[controller] handle accpet nested attributes for class group assign
[model] has_many through handle join model /students - class group
[view] nested form for class group assignment
[spec/model] xit validation for foreign_key because is used association presence for nested_attributes
[fix] model validations
[model] reject if class_group_enrollments attribute class_group_id is empty
[spec/features]check when studnets are create with or without class group
Merge pull request #256 from Genshin/fix_class_group_assign_on_student_create

Fix class group assign on student create