getPlaylist(playlistId: string | Playlist,requestingUserId: string | null): PlaylistWithDumps
Returns a playlist with the dumps requestingUserId may see in it, in
order, for GET /api/playlists/:playlistId, and for
GET /api/users/:key/playlists/:slug once it has found the playlist by its
path, for the same viewer. A null requestingUserId is a signed-out viewer.
Its dumpCount is the number of those dumps.
playlistId: string | Playlist