make distinction between valid and invalid datasetdesriptions

[coret]

A datasetdescription is only added to the dataset register is it's valid. After being added, the datasetdescription can become invalid, due to changes to the datasetdescription or requirements. The current search function of the demonstrator just shows all datasetdescription. To help users searching and stimulate publishers, we do want to make the distinction valid/invalid.

The search form will get a checkbox with the label "Only valid datasetdescriptions", which is checked by default.

When this checkbox is checked, search results will only include valid datasetdescriptions.

[coret]

Piece of SPARQL which has to be added for the search for valid datasetdescriptions is:

    ?url a schema:EntryPoint ;
         schema:about ?dataset ;
         schema:status ?status .
    FILTER (?status = 200)

But, then we only have 695 valid datasetdescriptions (out of 8192) at the moment, @ddeboer ?

[ddeboer]

That seems about right. Compare the number of orphan datasets, which no longer have a valid registration.

To pick an example registration: Spinque’s datacalog became invalid (schema:validUntil) at 2023-10-16T21:02:04.942Z. This was reported already at netwerk-digitaal-erfgoed/dataset-register#352 (comment).

To keep the reference between datasets and their registration when the latter becomes invalid, I added netwerk-digitaal-erfgoed/dataset-register#825.