Add an index to the story notification table

Summary: Fixes T8433. Add an index to the `phabricator_feed.feed_storynotification` table. Primarily, this speeds up object destruction.

Test Plan: Ran `./bin/storage upgrade` and saw the migration apply cleanly.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8433

Differential Revision: https://secure.phabricator.com/D13164

src/applications/notification/storage/PhabricatorFeedStoryNotification.php

@@ -25,6 +25,9 @@ final class PhabricatorFeedStoryNotification extends PhabricatorFeedDAO {
         'userPHID_2' => array(
           'columns' => array('userPHID', 'hasViewed', 'primaryObjectPHID'),
         ),
+        'key_object' => array(
+          'columns' => array('primaryObjectPHID'),
+        ),
       ),
     ) + parent::getConfiguration();
   }