adds 'most commented' order to debates

app/controllers/debates_controller.rb

@@ -82,7 +82,7 @@ class DebatesController < ApplicationController
     end
 
     def parse_order
-      @valid_orders = ['created_at', 'score']
+      @valid_orders = ['created_at', 'score', 'most_commented']
       @order = @valid_orders.include?(params[:order]) ? params[:order] : 'created_at'
     end