Support broadMatch/hierarchical gazetteers

[rsimon]

Model-wise, we can add is_part_of or skos:broadMatch relations to items, to indicate a parent-child relationship. (This is different to skos:closeMatch and skos:exactMatch as it won't lead to conflation.) But the implementation is unfinished. What we want is...

1. Items linked to a places that has a broadMatch should show up as results to their parent match
2. The Linked Data view should pull broadMatches out of the system and render them into the graph
3. (Potentially) search result list and selection box should indicate the existence of a broadMatch (and show a formatted link).

Problem re 3.: how to distinguish between broadMatches that are actually resolvable in the system vs. those that aren't? Potentially, we'd need to ping this on demand from the UI. The other alternative would be to pack this into the ingest (which would make it more complex)

[rsimon]

P.S.: gazetteers currrently making use of broadMatch are: Cyprus, Libya, DEFC.