concepts hierarchy (API service)

this adds an API service at `/hierarchy` to retrieve a given concept's surrounding hierarchy

because it's "just an API service", there's no UI integration - i.e. `/hierarchy` is not linked to from anywhere (while there is an HTML representation, that's mostly for the dev's benefit)

`/hierarchical_concepts` remains unaffected by this, as it serves a different purpose (rendering only one level at a time)

note that this can become *quite* expensive: since we have to traverse the hierarchy one step at a time, there'll (inevitably?) a ton of database queries

careful review would be appreciated