Ability to update index.json without building app

[sverhoeven]

The web app now reads the index.json from the src when its bundled during build. So just before build the scrape script is executed.

It would be nice to be able to build the app and run the scrape separated in their own GH action workflow.
The app will then not be build when the scraper generates a new file.

For this to work we should move index.json to https://github.com/jasp-stats-modules/modules-registry where it is hosted as https://jasp-stats-modules.github.io/modules-registry/index.json.

The app is able to fetch the index.json from there and show a loading spinner. We should take care that local app development is possible with a local index.json file.
In #20 spinner and ?c=index.json was added.

[sverhoeven]

Alternatively use publish to gh-pages see #21