Create dashboard panels inline on dashboards

Summary:
Fixes T5308.

• Allows you to create a panel directly on a dashboard.
• Also, include existing panels with a select instead of a text field. This won't scale as well but should be fine for now, and is way easier to use.

Test Plan: See comment.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5308

Differential Revision: https://secure.phabricator.com/D9501