Don't show the street map at close zooms for now

Turns out showing the street map at close zooms doesn't provide a good experience:
* Not enough features are shown in many areas, so it's unclear to users that they're looking at a street map
* Parcel layer outlines are too light on the Bing tiles, so we would need to restyle them 
* We can't use customizable Mapbox tiles, since they only go down toZ19