fully implement sio:describes or similar property and generate RDF accordingly

[graybeal]

As suggested by #783, it should be possible to specify an IRI for the original entity being described by a metadata instance. With that information CEDAR could generate RDF that correctly associates the user-provided metadata with the original entity. (Instead of using the instance IRI as the subject for both the artifact metadata, and for the original object, as originally described in #695.) The triples in the RDF should look like
original-entity-IRI field-property value
instead of
instance-metadata-IRI field-property value

To do this the following work is needed:

1. the template editor lets the author specify that instances should collect this information from the metadata creators
2. the metadata editor presents a field for collecting this information (possibly in the metadata location for identifier string in templates)
3. the appropriate property must be decided upon
4. the metadata editor generates JSON-LD that represents this model correctly/consistently with the RDF
5. the metadata editor generates RDF that includes the original-entity-IRI as the subject for all user-provided metadata
6. user guides need be updated

[graybeal]

In discussion we considered a temporary solution involving adding a field or object to each template that would specify the original-entity-IRI with an isMetadataOf property. That is addressed in #1209, as it does not meet the needs that the above description addresses and should be discussed separately.