Applies req to a playlist for PUT /api/playlists/:playlistId, and returns it as it now is. A new slug goes through changeSlug, first, and a moderator is exempt from its rate limit; a new title leaves the slug as it is. canModerate lets someone other than the owner edit it. If the playlist is public, it notifies, in the owner's name, the users the description mentions. When it goes public, its followers are notified of its public dumps they weren't yet told of. It links the description's attachments and tells clients of the change (broadcastPlaylistChange).