PERF: add index to make finding new topics faster

db/migrate/20150301224250_create_suggested_for_index.rb

@@ -0,0 +1,6 @@
+class CreateSuggestedForIndex < ActiveRecord::Migration
+  def change
+    add_index :topics, [:created_at, :visible],
+                where: "deleted_at IS NULL AND archetype <> 'private_message'"
+  end
+end