Refactor / use ActiveRecord association operator

Using << here replaces manual foreign key assignment as well as the explicit save call.

Author: aaronskiba

app/models/plan.rb

@@ -259,8 +259,7 @@ def self.deep_copy(plan)
     plan_copy.identifier = plan_copy.id.to_s
     plan.answers.each do |answer|
       answer_copy = Answer.deep_copy(answer)
-      answer_copy.plan_id = plan_copy.id
-      answer_copy.save!
+      plan_copy.answers << answer_copy
     end
     plan.guidance_groups.each do |guidance_group|
       plan_copy.guidance_groups << guidance_group if guidance_group.present?