Release v4.104.0

Changed

• Reducer and specification artifacts in a render plan now also list their declaring slice among their sources, so a consumer such as Studio's slice Preview can find a slice's files by filtering on the slice id. Existing sources are kept, and the slice of an event a reducer reads never becomes an owner. Plan digests change; file contents don't. The render plans reference now lists which artifacts each slice owns. (part of #400)