Add facet for work-nodes

[lpugin]

Work-nodes are linking sources through an internal 930 marc field, with $0 being an external authority ID (e.g., GND).

We would need this field to be searchable in the API so we can find all the sources relating to one work-node through the specified $0 with a (xxx) prefix.

A next step would be to make the work-nodes visible and browsable from the source display. We would need two links, one to the external authority record, and one for seeing all the sources referring to it. See the mock-up below.

(The mapping for $0 (xxx) prefixes is given here. GND is the only external authority source used so far.)

The link to all the sources will act as a facet applied from the link.

A next step would be to have the facet visible and de-selectable in the UI. See the second mock-up below