Required fork of leaflet-hotline with a new option to disable the
gradient display.
This is was done so that short stretches of very bad surface adjacent to
very good ones are visible.
Also note the eslint-disable-line for this compat warning:
URLSearchParams is not supported in Safari 7, op_mini all, IE 10, android 4.1
don't seem relevant today because those are EOL for a long time now.