DataPackageManager: A new data package revision should duplicate its predecessor's package-level (metadata) thumbnail

[servilla]

When update a data package with a new revision, one of the final transactions of the upload process should be to duplicate a predecessor's package-level (metadata) thumbnail if one exists. This sets an existing thumbnail by default, while allowing the owner to modify it by deleting, then creating a new thumbnail.

[servilla]

This action should only be allowed for the package-level thumbnail: consistency between data entity thumbnails is not easily possible.