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/contributor_creation.rb

@@ -4,11 +4,15 @@ module ContributorCreation
   extend ActiveSupport::Concern
   include Contributor
 
+  ALLOWED_CONTRIBUTOR_TYPES = [InternalUser, ExternalUser, ProgrammingGroup].map(&:to_s).freeze
+
   included do
     belongs_to :contributor, polymorphic: true
     alias_method :user, :contributor
     alias_method :user=, :contributor=
     alias_method :author, :user
     alias_method :creator, :user
+
+    validates :contributor_type, inclusion: {in: ALLOWED_CONTRIBUTOR_TYPES}
   end
 end