Improve (sub-)type filtering

[acka47]

Currently, we only allow filtering by first-level types Person, CorporateBody, ConferenceOrEvent, Work, PlaceOrGeographicName, SubjectHeading, Family.

One adjustment should be that the filter section adjusts when clicking on a type: Either the not-clicked first-level types shoud disappear (and the sub-types should be shown, see below) or the not-clicked types should be greyed out.

We should also enable filtering by the subtypes in these two ways:

1. Add a "+" sign to first-level types with sub-typing so that people can klick on it and the sub-types are shown. (E.g. for "Place or Geographic Name" "Building or Memorial", "Administrative Unit", "Fictive Place" etc.)
2. Filtering by a first-level type should yield filtered search results with sub-facets opening up

[fsteeg]

Deployed drill down in type facet to test: http://test.lobid.org/gnd/search