Wikidata music cards: use RWO URIs from $1 if available

Wikidata music cards: use RWO URIs from $1 if available

If the bib record contains a $1 subfield that points to a Wikidata Real
World URI, such as http://www.wikidata.org/entity/Q709024, surface that
in the author's RDfa as a schema.org/sameAs relation.

For music cards, the presence of a Wikidata RWO URI enables us to skip
searching by name and serve up the data directly based on the RWO URI.

The explicit RWO URI avoids disambiguation problems as arises with names
such as "Alexander Young" who is both a Scottish musician and an English
tenor, and sets the stage for expanding the Wikidata entity card
functionality outside of the domain of music.

Signed-off-by: Dan Scott <dan@coffeecode.net>