Add ability to query dashboard panels by paneltype
Summary: Pretty basic, but you can now search panels by type (query, text, tab).
Test Plan: Searched for a few different types of panels, results look correct
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11782