MVP for SNS/UMTHES integration to fetch related terms.

[adamreichold]

This probably needs at least per-term requests with caching and locally building the union over the currently queried terms to be useful in the long term, but it works well enough to demonstrate the idea.

[adamreichold]

Just dumps the related terms below the facets and allows them to be added to the query, i.e. first

and then after clicking on "Alster"

IMHO, the main usability issue here is how to include this in the search box, e.g. replace the term that it is related to, add a new term that can match or must match.

[adamreichold]

@jakobdeller As discussed, I extended this with caching so that for each term, we will only fetch similar terms once per day. I think it is therefore ready to include in the main branch after review.