Make Dashboards and Panels Flaggable

Summary: Ref T8888, makes these two objects flaggable.

Test Plan: Set Flag, See Flag.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8888

Differential Revision: https://secure.phabricator.com/D13652