Persist OriginalRecord

This is a substantial rewrite of OriginalRecord to allow them to persist to LDP.
Is this the same assumption as what the [`it 'sets rdf_subject'` spec](https://github.com/dpla/KriKri/pull/16/files#diff-6f8d6809e6e1395da9f72c3d44d864c3R90) is checking?