Make PhabricatorBotFeedNotificationHandler work with new PhabricatorApplicationTransactionFeedStory

Summary: Depends on D8521

Test Plan: ran with handler enabled and phabot posted stories

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8524