Serialize UMTHES tag with their label for the JSON API

For external consumers, it is helpful if we resolve UMTHES tags for them and provide the associated label instead of only the ID. (But we should still provide the ID so that e.g. the UI can produce links towards SNS.)

The resulting API responses will look something like

"tags": [
    {
        "Umthes": {
            "id": 6649961,
            "label": "Wasserrahmenrichtlinie"
        }
    }
],

While I was here, I also fixed an error in the OpenAPI specification of the origins field (which is a list, not a single value) and removed some unnecessary attributes from the definition of OldDataset. (Unnecessary because this type is used only to deserialize the previous binary format and never used e.g. to (de)serialize JSON.)