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