add educational stages relation

app/models/collection.rb

app/models/concerns/filterable.rb

app/models/educational_stage.rb

app/models/stage_relation.rb

db/migrate/20161026134028_create_subject_relations.rb

db/migrate/20161027134827_add_index_to_taggings.rb

db/migrate/20161027141322_create_stage_relations.rb

lib/tasks/import/educational_stage.rb

lib/tasks/import/subject.rake

test/fixtures/educational_relations.yml

test/models/stage_relation_test.rb