Add "Searchworks Preview" link for an item published to PURL

[LynnMcRae]

We have a series of related links in the upper left of the detail display -- to PURL, FoXML, SOLR. It would be nice to have Searchworks there as well. The link would be be there only for non-dark items that have been accessioned (have a Purl) (or maybe always ... we have a blind PURL link, verifying whether something is or isn't there). Note that unlike the link to Searchworks itself, the link to preview is always by druid.