Linked data: link out to LoC relators from record summaries

* Evergreen master

Right now we already look to relator codes in 1xx and 7xx fields to find the right label to use for "Added author" entries in templates/opac/parts/record/authors.tt2

We can go one step further and include links to the id.loc.gov vocabulary to more accurately identify (for machines) the nature of the contribution that the given added entry has made as a second value of the @property; for example, instead of:

<span property="contributor" typeof="Person"><span property="name">Maher, Paul,</span> <span property="birthDate">1963</span>- (<span property="description">Editor</span>). </span>

we can and should do:

<span property="contributor http://id.loc.gov/vocabulary/relators/edt" typeof="Person"><span property="name">Maher, Paul,</span> <span property="birthDate">1963</span>- (<span property="description">Editor</span>). </span>