Switch CDN topojson maps to UN sourced data

[camdecoster]

Description

New maps based on UN geodata were released in 3.1.0-rc.0. We should switch the topojson maps on the CDN to these new maps instead of the current (legacy) maps.

Notes

• The legacy maps are served as the default at https://cdn.plot.ly/<map_name>
• The UN based maps are available at https://cdn.plot.ly/un/<map_name>
• The Libraries team should have a discussion about when to make this change as it will affect anyone using the maps via the CDN
• Example links

  Legacy	UN
  https://cdn.plot.ly/africa_50m.json	https://cdn.plot.ly/un/africa_50m.json