Merge pull request #270 from Genshin/class_groups

Class groups
[frontend] Refactor preset chooser_fields
[frontend] Clean students_controller
[core] Remove StudentConfig
[core] Remove default db data for StudentConfig
[frontend] Fix students index view so if no Preset it still works
Try to fix some failures
[frontend] Make student_chooser to use Preset
[frontend] Remove views/shared/person_fields partial
Use preset for name, surname, birth_date, gender in students index
[frontend] Add specs for name, surname, birth_date, gender presets in students index
Add show_middle_name to chooser_fields preset
Add specs for show_code preset
Add specs for show_admitted preset
Add specs for show_primary_contact preset
Add specs for primary_address preset
Add specs for personal_information
Add specs for personal_information
Add show_graduated to chooser_fields preset
Add show_enrollment_status to chooser_fields preset