Duplicate user fix use sql in migration

This breaks the dependency on the User class and it's `activated?` method.

This means the migration is only aware of how the data is stored in the database at this specific point in time before and after; and we won't break our ability to migrate up if we rename `activated?` to `active?` at some point in time.

I did not test this with sqlite, so we'll see what Travis-CI says.