Check polymorphic type in validations

* Previously, some models had "duplicated" `belongs_to` associations, which were now removed (and replaced by the `include Creation`).

app/models/concerns/creation.rb

@@ -3,9 +3,13 @@
 module Creation
   extend ActiveSupport::Concern
 
+  ALLOWED_USER_TYPES = [InternalUser, ExternalUser].map(&:to_s).freeze
+
   included do
     belongs_to :user, polymorphic: true
     alias_method :author, :user
     alias_method :creator, :user
+
+    validates :user_type, inclusion: {in: ALLOWED_USER_TYPES}
   end
 end