Better integration of UMTHES-based tags

While !107 (merged) efficiently stores tags sourced from UMTHES, we currently store only a text definition of those but no facets. We should have a structure tag definition that uses the RDF serialization of the concept information to attach multiple paths into the topic facet, for example

1. https://sns.uba.de/umthes/de/concepts/_00652832.rdf
2. https://sns.uba.de/umthes/de/concepts/_00025264.rdf
3. https://sns.uba.de/umthes/de/concepts/_00049274.rdf

is the chain from "Meeresstrategie-Rahmenrichtlinie" over "Umweltrecht" to "[Umweltpolitik, Umweltschutzgesetzgebung]".

Since fetching this hierarchy will involve quite a few HTTP requests, we should only when a title changes (which we know without another HTTP request) and maybe even cache intermediate responses during a single harvest run.

Furthermore, we should drop our custom tags where they can be replaced with references into UMTHES (like those for the European directives on water bodies).