Allow changing the base color of the globe in the Cesium map

[robyngit]

When all of the layers are hidden, the base color of the map is a bright blue by default:

We should allow users to change the globe colour. This could be useful when vector data is visible but there is no imagery layer behind it; users may want to change the background below the vectors:

This option could be placed in a "settings/ map appearance" tab, or we could display "background color" as the bottom-most layer in the layer list (and disable hiding it)

[mbjones]

This makes sense and is a good idea. But I think it is lower priority than many of our other identified features for the Cesium map.