Garbage collect old notifications

Summary: Fixes T8473. Adds garbage collection to the phabricator_feed.feed_storynotification table.

Test Plan: Reduced the TTL to a really small value and ran the trigger daemon. Saw feed notifications removed from the database.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T8473

Differential Revision: https://secure.phabricator.com/D13233