Removing a model prevents tidus from clearing the view during next migration

[tobischo]

If you remove a model in order to remove the database table with the next migration, tidus is not capable of removing the view. This is caused because tidus checks the models and not all the previously existing views (because they may have a different origin than tidus). Currently this can be solved using force: :cascade in the migration but we might want to consider changing the way view clearing works for this scenario