Passed to SlugField by DumpEdit's and PlaylistEditForm's forms,
which send the slug only when it differs from saved.slug.
prefix: string
The page's path up to the slug, such as /@bob/dumps/.
saved: { title: string; slug: string; }
The item as saved, whose slug a title put back to its own restores.
optional
inputClassName: string
For a form outside .form, whose inputs take their own class.