Add a unique constraint and foreign keys

Also this was the first time I ran `rake db:schema:dump` in a while cause i forgot about it. It looks like takes care of adding in the schema_migrations table, which _I think_ means that https://github.com/heroku/telex/pull/24/files#diff-02b49c97a1c0f2b019fdbba32b895c81L8 is unnecessary, as long as db:schema:dump is ran after each migration. Should that be done by pliny?