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.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10145

Differential Revision: https://secure.phabricator.com/D17351