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