Support toggling of multiple basemaps

[goldpbear]

We'd like to be able to toggle among several different basemaps without loading un-toggled basemaps in the background, and without hiding other layers when a basemap is toggled. This seems to be the way the satellite layer works already. Maybe we can generalize the logic used to show that layer.

[goldpbear]

I think the satellite visibility is handled here, in the views/map-view.js file.