Fix invalid submit_uri when creating dashboard panels

Summary: When creating dashboard panels, the submit_uri is invalid since the panel has not been saved to the database yet (and therefore doesn't have an ID). This resulted in a 404 when trying to submit the form to /dashboard/panel/edit//

Test Plan: Created a dashboard panel and the panel was created successfully

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9629