R1:0387d626325c

Add Dashboard typeaheads

Summary: Builds a basic typeahead for Dashboards and Panels

Test Plan: `/typeahead/browse/PhabricatorDashboardPanelDatasource/`

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17064