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/exercise_collection.rb

@@ -1,12 +1,12 @@
 # frozen_string_literal: true
 
 class ExerciseCollection < ApplicationRecord
+  include Creation
   include TimeHelper
 
   has_many :exercise_collection_items, dependent: :delete_all
   alias items exercise_collection_items
   has_many :exercises, through: :exercise_collection_items, inverse_of: :exercise_collections
-  belongs_to :user, polymorphic: true
 
   def collection_statistics
     statistics = {}