Change source of index field listed_in_ss

[nichtich]

Index field listed_in_ss is currently filled from JSKOS field partOf to indicate a concept schemes is listed in a registry. This assumed semantics of partOf is going to be removed. The connections between concept schemes and registries will instead come from Registry object field schemes.

Requires #41 to get registry data and update index field listed_in_ss.

Support of legacy query parameter partOf should be dropped as well but only after gbv/bartoc.org#266 has been done.

[nichtich]

Also requires changes in the database.