geoJSON layer: handle hover, improve click handling, use icons

[qualitymanifest]

• Added hover handling. Throttled the hover event so that it doesn't blow up slower computers Specified an interactive layer, which allows hovering to work properly
• Changed click radius for computers. Once hovering was implemented it was clear that the click radius was way too wide for precise pointers. User agent sniffing might not be a guarantee but it is probably the best option, short of waiting for a touch event and changing the click radius.
• Removed the coordinate correction hack. A bunch of the points will appear in Antarctica until the coordinates are corrected in the database.
• Close popups when clicking outside of feature
• Calculate the closest feature to a click
• Added ability to use images instead of built-in mapbox pins, and added a couple sample icons to show how it works. See relevant issues Icons for food site types #36 regarding icons and Location types should be standardized food-access-map-data#19 regarding location type attributes
• Added min & max zoom values to improve performance
• Trimmed unnecessary info out of geoJSON to improve map performance
• Updated react-map-gl. Fixed some issues and doesn't seem to have any problems with backwards compatibility.