search: fix order:note, order:comment_bumped to use indexes.

db/migrate/20170519204506_fix_last_noted_at_index_on_posts.rb

@@ -0,0 +1,11 @@
+class FixLastNotedAtIndexOnPosts < ActiveRecord::Migration
+  def up
+    Post.without_timeout do
+      remove_index :posts, column: :last_comment_bumped_at
+      add_index :posts, :last_comment_bumped_at, order: "DESC NULLS LAST"
+
+      remove_index :posts, column: :last_noted_at
+      add_index :posts, :last_noted_at, order: "DESC NULLS LAST"
+    end
+  end
+end