Notifies the followers of each playlist of the dumps in it that became visible to them: a playlist's followers are told of a dump once both are public. The dump service calls it with dumpId when a dump goes public, for each public playlist holding it; the playlist service with playlistId when a playlist goes public, for each public dump in it, from the bottom of the playlist up. A dump the followers were told of before either went private isn't notified again. Adding a dump to a public playlist calls notifyPlaylistFollowersNewDump directly.