Fixed losing full size feed for maniphest task creation

Summary: After D2571, feed for maniphest task creation was being generated non-full-size. Fixed this by properly getting the maniphest action from story data.

Test Plan: Feed seems to work fine and the feed for task creation is full-sized.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: allenjohnashton, ddfisher, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2691