default result sorting to quiz submission descending

completes #85

db/migrate/20161204220527_add_completed_at.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+class AddCompletedAt < ActiveRecord::Migration[5.0]
+  def change
+    add_column :candidates, :completed_at, :datetime, after: :completed
+
+    Candidate.reset_column_information
+    Candidate.where(completed: true).each do |candidate|
+      candidate.completed_at = candidate.updated_at
+      candidate.save
+    end
+  end
+end