Add road completeness in Gap Detection app

[cgiovando]

Similarly to the current building/built-up area gap estimation, we could use ML-derived road layers to provide a measure of completeness for highway=*

This was done at country level: https://blog.mapbox.com/how-complete-is-openstreetmap-7c369787af6e but the app doesn't seem to work anymore, only the list here https://gist.github.com/tcql/0d7ad9b32afbea76f615

Some of the ideas in this article could be applied to develop the completeness index https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0180698