Add user (ID and type) to proxy exercise

This is required for the existing policies to work

db/migrate/20181119161514_add_user_to_proxy_exercise.rb

@@ -0,0 +1,9 @@
+class AddUserToProxyExercise < ActiveRecord::Migration[5.2]
+  def change
+    add_reference :proxy_exercises, :user,  polymorphic: true, index: true
+    add_column :proxy_exercises, :public, :boolean, null: false, default: false
+
+    internal_user = InternalUser.find_by(id: 46)
+    ProxyExercise.update_all(user_id: internal_user.id, user_type: internal_user.class.name)
+  end
+end