db: add created_at indexes on post_appeals, artist/note/wiki_page versions,

This optimizes queries on /moderator/dashboard that filter by creation date.

db/migrate/20170319000519_add_created_at_index_to_versions.rb

@@ -0,0 +1,10 @@
+class AddCreatedAtIndexToVersions < ActiveRecord::Migration
+  def change
+    ActiveRecord::Base.without_timeout do
+      add_index :note_versions, :created_at
+      add_index :artist_versions, :created_at
+      add_index :wiki_page_versions, :created_at
+      add_index :post_appeals, :created_at
+    end
+  end
+end