Multi-country visualizations for monthly activity and features edited

[gracedoherty]

We'd like Open Cities Africa to display aggregated data for all 11 cities in single charts or indicators. For example, the OSM Monthly Activity chart on https://opendri.org/project/uganda/ shows all activity for the country of Uganda. We want the same visualization, but summing up monthly activity across the 11 OCA cities.

Similarly, Buildings Edited would be the sum of all buildings edited across all 11 cities, etc.

I can't see a simple way to do this for the map Before/After visualization, but happy to hear ideas.

The visualizations chosen should align with those decided on for each city, per this issue: GFDRR/opencities#20.