Add a "start with a template" option when creating dashboards

Summary: Ref T5317. This primarily makes it easier for new administrators to build a dashboard for the first time, without going too crazy on technical complexity.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5317

Differential Revision: https://secure.phabricator.com/D9651