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/community_solution.rb

@@ -1,12 +1,15 @@
 # frozen_string_literal: true
 
 class CommunitySolution < ApplicationRecord
+  ALLOWED_USER_TYPES = [InternalUser, ExternalUser].map(&:to_s).freeze
   belongs_to :exercise
   has_many :community_solution_locks
   has_many :community_solution_contributions
   has_and_belongs_to_many :users, polymorphic: true, through: :community_solution_contributions
   has_many :files, class_name: 'CodeOcean::File', through: :community_solution_contributions
 
+  validates :user_type, inclusion: {in: ALLOWED_USER_TYPES}
+
   def to_s
     "Gemeinschaftslösung für #{exercise}"
   end