Provide VOID data for crawled datasets

## Description

As a user, I would like to have VOID data for a dataset from which I do not know its exact position. A web service should be able to offer this information, e.g., based on the label of the dataset or a part of its URI.
(See the discussion at https://lists.w3.org/Archives/Public/semantic-web/2018Apr/0087.html describing that a common way of accessing this information about datasets still is missing.)

## Solution
- [ ] Gather VOID information (classes, properties, counts, ...) from crawled datasets (most probably as a post processing step)
- [ ] Provide VOID data (together with URIs at which datasets have been found (DCAT)) in a SPARQL endpoint
- [ ] Provide an easily accessible REST API for querying this data for a given dataset

Start by reviewing the crawler's post-processing flow and the existing SPARQL endpoint and REST API entry points. Define how VOID information and DCAT dataset URIs should be gathered and exposed, including the supported dataset lookup inputs; done means all three solution checklist items are implemented and queryable.

Written by the indexing model from the issue text.