* Add a guard for email message content that does not have an html tag
* Return empty string if content is nil
* Add tests for handling content with and without html tags and nil
* Add FK between ahoy_messages and feedback_messages
* Add FKs between articles.second_user_id, articles.third_user_id and users
* Fix associations and specs
* Add more validations
* Remove FKs to articles.second_user_id and .third_user_id as they are about to be removed