Include interactive choropleth maps in web app

[carlhiggs]

Expanding on #344 and #375, related to #402 and #408

For users with regions that have been analysed, and are displayed on a map, there should be an option to go beyond viewing region summary statistics to select a spatial indicator and view its distribution across the study region at the standard configured grid scale.

This option could be available as select menu within the popup indicator summary for overall region statistics.

It could be constructed using the ghsci.Region.choropleth Folium based method of constructing a leaflet map.

Later, we could consider adding a button for users to save the generated map as html or png.