Implement a more seamless transition between 2D and 3D visualizations

[jimmyangel]

Right now, switching between these two modes require page reloads and using different libraries, Leaflet for 2D and Cesium for 3D. Cesium supports 2D visualization, so an easy way to do it is to just use Cesium instead of using two separate libraries. But I have been hesitant to do that because of the hefty resource requirements of Cesium, especially noticeable in mobile devices – it is good to have a “lite” path that works well on mobile. After the upgrade above, I will go ahead and re-evaluate the decision.