db: add updated_at index on wiki pages.

The wiki pages sidebar is ordered by updated_at. The sidebar is present
on all wiki pages, so this improves load times for all wikis.

db/migrate/20170329185605_add_updated_at_index_on_wiki_pages.rb

@@ -0,0 +1,7 @@
+class AddUpdatedAtIndexOnWikiPages < ActiveRecord::Migration
+  def change
+    WikiPage.without_timeout do
+      add_index :wiki_pages, :updated_at
+    end
+  end
+end