Implement event-based interaction between CloneImagesOrVideos and biigle/largo

[mzur]

The CloneImagesOrVideos job currently has a hard dependency on biigle/largo. While it uses class_exists() and works without the module, too, this is bad style. The job already emits a volume.cloned event. Update biigle/largo to pick up this event and initiate the processing of the cloned files. Then the hard dependency of CloneImagesOrVideos can be removed.