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Reverts LaunchAcademy/south_park_reviews#23


It's a good idea to validate that the `role` is included in a specific set of roles so we don't end up with multiple variations of the same thing or unintended roles. It's also helpful when querying/indexing to make sure that the roles are already always in a specific case.

```ruby
ROLES = %w(admin default)

validates :role, inclusion: { in: ROLES }
```