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
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