R1:baf4d451aabb

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…