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