Add storage and read logic for workboard card cover photos
Summary:
No way to set photos yet, but if you magic them in they work.
Primarily, this consolidates rendering logic so the move + edit + view controllers all run the same code to do tags / cover photos.
Test Plan: {F1095870}
Reviewers: chad
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D15201