Add Rights Statement as metadata

[kerchner]

Ref gwu-libraries/scholarspace-hyrax#106

This is a required field in GWSS, but currently the ingest_work task will provide a default. This ticket is to add rights statement as a metadata field so that a value other than the default can be used,

Documentation should provide recommended defaults and point to list of valid values in GWSS.

[kerchner]

Currently, Works are being loaded in GWSS with the following defaults, so it's okay if these aren't specified for now, but these defaults should be documented:

• Rights Statement = http://rightsstatements.org/vocab/InC/1.0/
• License = http://www.europeana.eu/portal/rights/rr-r.html