Add hovercard on/off option to PhabricatorFeedStory

Summary: Defaults hovercards off everywhere feed stories are shown. I tried to find where to put this in so /feed/ could display them, but got horribly lost and confused in SearchQueryLandView

Test Plan: turn hovercards on and off, inspect elements.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6757