Allow panels to appear on dashboards

Summary:
Ref T3583. Adds edges, query relationships, etc. Lots of debugging/temporary UI.

My general intent here is to use edges to track where panels appear, and then put additional data on the dashboard itself to control layout, positioning, etc.

Dashboards don't actually render yet so this is still pretty boring.

Test Plan:

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3583

Differential Revision: https://secure.phabricator.com/D8916