Minor fixes to maniphest feed/notification

Summary:
Added renderNotificationView() abstract function to PhabricatorFeedStory base class.
Fixed duplicate line in PhabricatorFeedStoryManiphest class.
Fixed spacing/formatting in ManiphestTransactionEditor.

Test Plan: No functional changes

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: allenjohnashton, ddfisher, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2698