Don't allow duplicate panels on dashboards
Summary: Fixes T10145. I went with "don't add two panels", since panels are easy to create, I expect this to be a reasonable limit until we have better use cases.
Test Plan: Try to add the same panel twice, get error. Add panel normally fine, move panels fine, edit panels fine.
Reviewed By: epriestley
Maniphest Tasks: T10145
Differential Revision: https://secure.phabricator.com/D17351