Upgrade to react-map-gl@7

[gkovats]

For issue #306 : Upgrade react-map-gl

Some component props changed. Updated according to docs: https://visgl.github.io/react-map-gl/docs/api-reference/map

• Updated the webpack.mix.js file to force 1 output js / no code splitting
• Markers are pixels close to original, used Google maps for comparison and they look good zoomed far out and at they approach. Still, worth validation.
• Popup positioning is odd, won't flip around an offset. Example:

top, it's close to original, but when flipped to below, it drops offset distance from marker point.

They only have so many props, tried them all, this was as far as I got. Seems usable, but irksome.

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[joshreisner]

good observation about the popup positions, but nbd. i'm confident we'll fix it in time