add index for media objects

fixes CNVS-27288

Change-Id: I6042f62a40b2374216484d8ad715ce522b7a0941
Reviewed-on: https://gerrit.instructure.com/72238
Tested-by: Jenkins
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

db/migrate/20160216165757_add_media_objects_index.rb

@@ -0,0 +1,8 @@
+class AddMediaObjectsIndex < ActiveRecord::Migration
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def change
+    add_index :media_objects, :root_account_id, algorithm: :concurrently
+  end
+end