Rethink how subject codes are stored in Thoth

[rupertgatti]

Desire to be able to provide info about a subset of titles around the subject discipline category (rather than, say, the publishers) in a more aggregated level than specific subject codes. eg for all books in History - do we provide a 'lookup' package of category codes to filter by (BISAC - first letters HIS, Thema first letters NH ....).