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