avoid huge autocomplete payload

[stefanprobst]

currently we request all autocomplete results from /visualisations/entityautocomplete/, i.e. 5 MB of data.

also, it seems that the endpoint duplicates every result:

export interface HierarchyNode {
  group: "Institution" | "Person" | "Funktion";
  label: string;
  pk: number;
  // FIXME: Duplicates [label, group, pk].
  value: [string, string, number];
}