Create an empty state for dashboards

Summary: Fixes T5177. Not sure if checking for panelPHIDs is right, but seemed like a better choice than adding a new property on dashboard.

Test Plan: Create dashboard with no panels. Go to view dashboard. "view" page should have a placeholder that directs user to Manage Dashboard

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5177

Differential Revision: https://secure.phabricator.com/D9312