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/user_exercise_intervention.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class UserExerciseIntervention < ApplicationRecord
-  belongs_to :user, polymorphic: true
+  include Creation
   belongs_to :intervention
   belongs_to :exercise
 end