Renders /@:username/playlists/:slug, for anyone who may see the playlist: a private one only to its owner. usePlaylist loads it, by a former name or slug too, and keeps it live. Its owner, or a user with playlist:moderate, can drag its dumps into a new order and remove one, with a short window to undo, edit its title and description in place, and edit the rest or delete it in the editor that opens over its header at /@:username/playlists/:slug/edit. Both paths render the page at the same place, so opening the editor keeps the playlist loaded. Anyone else there gets the page, or is asked to sign in first.