Accessibility: basemap contrast

[aviannamiller]

Some of the basemaps we offer do not meet contrast guidelines for non-text content (at least 3:1, exceptions apply for map types such as heatmaps where color gradients are essential for map representation). The default basemap, CartoDB.Voyager, has about a 1.2:1 contrast with no map boundaries to provide sufficient distinction (as modeled in WCAG Technique G209: Example 3: Map with border boundaries
).

We should also set a contrast approved basemap as the default and assess all offered basemaps to determine which are above contrast minimums to suggest for use in a visible manner. If basemaps offered fail contrast minimums, we could consider replacing those if they cannot or will not be modified.

[jimsafley]

#81 changes the default basemap to OpenStreetMap, which covers the first request here. OSM has the road casings and boundaries that Voyager lacked. I assessed the other basemaps. A handful have essentially no content that meets the minimum, mostly the terrain and relief styles.

I'll add a one-line suggestion to the Base Map setting naming the two highest-contrast basemaps that don't need an API key. I'm not labeling basemaps individually in the form, mainly because "not recommended" is a strong claim to put in the UI, and regrouping them makes them harder to find.