Add Demographic Toggle Layers

[crvanpollard]

Add several demographic datasets as toggle base layers, Include:

• Total Popultion
• Employment
• Zero-vehicle Households
• DVRPC 2015 Land Use

Demographic Feature Service
https://services1.arcgis.com/LWtWv6q6BJyKidj8/ArcGIS/rest/services/AccessScore/FeatureServer/11

Land Use Vector Tiles
https://tiles.dvrpc.org/data/dvrpc-landuse-2015/#13.87/39.97184/-75.15899

[crvanpollard]

Land Use Style from PA TIP
https://github.com/dvrpc/pa-tip/blob/master/src/components/map/layers.js