Renders /@:username/dumps/:slug/edit, for the dump's poster and moderators; anyone else signed in gets an error rather than a form the API would refuse to save. The dump comes from GET /api/users/@:username/dumps/:slug, which answers a former slug too, and the URL then takes the current one. The thumbnail, the preview refresh and the deletion apply at once, and the form's other fields on "Save", which goes back to the dump's page, at its new path if the slug changed. A deletion goes to the index.