query-string search should work with vocabulary token instead of value

[jackahl]

As the vocabulary in a token is the proper unique identifier for that term it should be the thing that is stored in in the volto frontend. Currently querystring uses the value of that term to assemble the search, which potentially could lead to issues as these can theoretically be of datatypes that are not reasonable to store in the frontend (e.g. python objects). Therefor especially when used with restapi and volto querystring searches should mostly rely on the vocabulary token.

The prerequisites to implement this are already available in #107.