Set up label lookup file for GND entries (RPB-202)

Based on Allegro data; GND entries in rpb-authorities have been
removed from Strapi data in https://jira.hbz-nrw.de/browse/RPB-172

`cat conf/RPB-Export_HBZ_SW.tsv | grep -E -w '^[0-9]+n?[0-9X]?'`

Possible approaches to include new GND entries used in RPB records
might be using the Strapi labels (add `afterFindMany` hook and make
the labels non-private, see [1] for an implementation; all those
label lookups might significantly impact performance though), or
showing the labels dynamically in the web OPAC only.

[1] https://github.com/hbz/strapi-rpb/pull/45/files