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