Add a rough "Chart" Dashboard Panel

Summary:
Depends on D20484. Ref T13279. Allows a chart to render as a panel.

Configuring these is currently quite low-level (you have to manually copy/paste a chart key in), but works well enough.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20485