Enforce presence of some/all fields in data `index.json` file

[alexdewar]

I've released that the schema for the index.json file isn't quite right as it stands.

There are two problems:

1. Users can omit any of the specified properties and the file will still validate
2. Users can add arbitrary additional properties

The second is less serious (though still potentially a source of user error), but the
first is important.

The solutions are:

1. Making use of the required keyword throughout the schema
2. Adding additionalProperties: false throughout

Yuck.