[web,catalog][s]: switch to sourcing list of datasets from new registry [1].

[1]: https://github.com/datasets/registry

* This had side effect that owner no longer exists so removed support for that.
[theme][xs]: change name to Datasets Project.
[content/about][xs]: tweak about page.
[catalog][s]: switch to using pre-prepared index.json to initialize the catalog.
[template][s]: sort out displaying the sources attribute.