Accessibility: basemap contrast

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.