[Discussion] Remove ObjItem gradient

Summary: Just wanted to play with this, removes the gradient 'cards' for a flat design.

Test Plan:
Tested various apps, workboards

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: hach-que, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9515