Add user (ID and type) to proxy exercise

This is required for the existing policies to work

app/models/proxy_exercise.rb

@@ -1,11 +1,16 @@
 class ProxyExercise < ApplicationRecord
+    include Creation
+    include DefaultValues
 
     after_initialize :generate_token
     after_initialize :set_reason
+    after_initialize :set_default_values
 
     has_and_belongs_to_many :exercises
     has_many :user_proxy_exercise_exercises
 
+    validates :public, boolean_presence: true
+
     def count_files
         exercises.count
     end
@@ -19,6 +24,11 @@ class ProxyExercise < ApplicationRecord
     end
     private :generate_token
 
+    def set_default_values
+      set_default_values_if_present(public: false)
+    end
+    private :set_default_values
+
     def duplicate(attributes = {})
       proxy_exercise = dup
       proxy_exercise.attributes = attributes