SCRUM#275: fixing on_delete for foreign key on attachments

app/controllers/v1/learning_objects/attachment_controller.rb

app/models/learning_object/attachment.rb

app/serializers/learning_object/attachment_serializer.rb

...5120824_update_learning_object_attachments_foreign_key.rb

...ove_learning_object_attachments_foreign_key_on_cascade.rb

...6_remove_attachment_id_for_learning_object_attachments.rb

...3124526_add_foreign_key_to_learning_object_attachments.rb