neo4j-graphql-js doesn't generate filters for custom scalars

[alastair]

We have a custom scalar type URL for url fields, but the bindings don't generate filters for them: neo4j-graphql/neo4j-graphql-js#340

This causes us problems when trying to filter on fields that are made up of a URL (#132).
Because we don't do any special validation or use this type for any specific reason, it will be easiest if we switch to String for these types.