Provide a 'no dashboards' fallback state if you can't add any
Summary: Ref T10390. Catch if the user doesn't have any dashboards they can edit and give them a helpful message instead.
Test Plan: Clean install, no dashboards, Click "Add to Dashboard" on ApplicationSearch results, see no dashboards message
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T10390
Differential Revision: https://secure.phabricator.com/D17528