R1:ea954c37e460

Add dashboard panel types

Summary: Ref T3583. These will be the primary class carrying panel implementations.

Test Plan:
{F149125}

{F149126}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3583

Differential Revision: https://secure.phabricator.com/D8912