Add admin level to country object served by /mobility/countries

[mikefab]

Task

Add admin level to country object:

{
  "country": "col",
  "source": "acme",
  "shapefile": "santiblanko",
  "path": "/mobility/sources/acme/series/santiblanko/countries/col",
  "version": "/api/v1",
  "admin_level": 2
}

Why do this

Depending on source, mobility data can only be aggregated to a certain admin level. For instance, world pop can be aggregated to admin level 2 gadm2-8 for DRC, while other sources can be more granular.

In order to know what shape file to use, that information must come with the country object.