Allow Dashboards to set an icon

Summary: Allows users set an icon (for reuse on upcoming home) for their dashboard based on 16 descriminating choices.

Test Plan: Create a new dashboard, set new icon. Edit an existing dashboard, set icon.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17042