Add user_type to RemoteEvaluationMappings

app/models/remote_evaluation_mapping.rb

@@ -2,9 +2,9 @@
 class RemoteEvaluationMapping < ApplicationRecord
   before_create :generate_token, unless: :validation_token?
   belongs_to :exercise
-  belongs_to :user, class_name: 'ExternalUser'
+  belongs_to :user, polymorphic: true
 
   def generate_token
     self.validation_token = SecureRandom.urlsafe_base64
   end
-end
+end