Basemap switcher

[tgilcrest]

We want to add a basemap switcher to the app to give users the ability to view basemaps underneath their maps. This task involves adding a basemap switcher and showing basemaps underneath the geounit lines, population labels, and district shapes. Note that because we won't have a transparency slider, we'll also have to modify how the districts appear (make them transparent) when a basemap is present. We'll use Mapbox basemaps for this. Links to be provided.

AC:

1. There is a basemap switcher in the top right corner that allows selecting from different basemaps
   • None (default)
   • Streets
   • Satellite
   • Topography
2. There is a preview thumbnail for each basemap option
3. When a basemap is selected, the districts colors become transparent, with a less-transparent color border on the boundary of the district.
4. Unlike the prototype, we should make the unassigned areas transparent (not translucent white). Note that for Mapbox GL, “transparent” is a color, so this could make it good way to drop it in as a color (and replace if we need to make it a non-transparent color).

Basemap selector (don't worry about creating the "Imagery" section for now):

Transparent districts:

[jfrankl]

I just opened a pull request that adds a basic basemap to the application #401. That will be suitable for launch.

In terms of adding a basemap switcher, I think we should spend some time thinking through our needs here. Getting the aerial imagery view to work well might be a big challenge, and I think it would be worth confirming its value. Instead of having 2 or 3 basemaps to choose from, we may want to explore an option where the user can choose between aspects of the basemap. For example: toggle street labels, toggle city labels, toggle hillshade.