Add unique constraints to FlaggableFlag, GroupFlag, OptOutFlag

Similar to https://github.com/jclem/active_record_rollout/issues/6.

| `OptOutFlag`                                     | `FlaggableFlag`                                  | `GroupFlag`                                    |
| ------------------------------------------------ | ------------------------------------------------ | ---------------------------------------------- |
| [`feature_id`, `flaggable_type`, `flaggable_id`] | [`feature_id`, `flaggable_type`, `flaggable_id`] | [`feature_id`, `flaggable_type`, `group_name`] |