Add a coverImage field to PHUIObjectItemView

Summary: Allows setting of an image to a Workboard card. (Hides from regular view, just in case).

Test Plan: Fake setting a Pholio Mock image.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15198