Upgrade React Mapbox GL and/or Mapbox GL

[toolness]

A bunch of the errors we're getting through Rollbar appear to be related to Mapbox GL, and I've noticed when quickly using my browser's back/forward buttons to navigate between WoW pages that some errors get logged related to this component too; for instance:

• In PropertiesMap.tsx it seems like our onStyleLoad is being called even once the component has been unmounted, which logs an error. It seems like not calling that callback if a React Mapbox component has been unmounted should be the responsibility of React Mapbox GL, not our code, but maybe I'm wrong.

• A Uncaught TypeError: Cannot read property 'send' of undefined is being logged from mapbox-gl.js and it's triggered by an XMLHttpRequest's onload event handler somewhere in that codebase.

Anyways, it looks like a new major version of React Mapbox GL exists (the one we're using, v3.8.0, is from February 2018) so maybe upgrading it will make some of these errors go away. And even if it doesn't, it will at least rule out the possibility that the errors are being caused by third-party bugs that have since been fixed.

[toolness]

We updated react-mapbox-gl in #420, so hopefully that will fix this...