Implement custom sensor markers

[caseycesari]

The markers for the sensors are custom, HTML-based icons that change based on data returned by the sensors.

This task is more implementing the markers, demonstrating that they are dynamic, but not for wiring them up to the actual data of the sensor. That will be for a later task.

Maybe helpful:

• https://blog.mapbox.com/custom-html-markers-in-mapbox-gl-js-d064868cb423
• https://docs.mapbox.com/help/tutorials/custom-markers-gl-js/