Add a "tabs" dashboard panel type

Summary:
Ref T4986. Allows you to create a dashboard panel out of dashboard panels.

bwahaha

Test Plan:
{F155472}

{F155473}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4986

Differential Revision: https://secure.phabricator.com/D9141