add settings table to DB schema

[sjanssen2]

The Qiita DB Schema file qiita_db/support_files/qiita-db.html is extremely helpful to understand the design of the database. I just found that there are additional tables (outside the qiita.* schema) that are also used by qiita, e.g. settings https://github.com/qiita-spots/qiita/blob/57d1b146729223da9daa286a1cc75db6f2923746/qiita_db/environment_manager.py#L351 Could you add these tables to the HTML overview?