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/remote_evaluation_mapping.rb

@@ -2,9 +2,9 @@
 
 # TODO: reference to lti_param_model
 class RemoteEvaluationMapping < ApplicationRecord
+  include Creation
   before_create :generate_token, unless: :validation_token?
   belongs_to :exercise
-  belongs_to :user, polymorphic: true
   belongs_to :study_group, optional: true
 
   def generate_token