Base layer picker icon is broken

[andre-carvalho]

File "layers-2x.png" missing from app/static/css/lib/leaflet/1.7.1/images/

In some OS/browser configurations, specific CSS class is used and background image is changed to leyers-2x which is missing from the repository.

.leaflet-control-layers-toggle {
  background-image: url(images/layers.png);
  width: 36px;
  height: 36px;
}
.leaflet-retina .leaflet-control-layers-toggle {
  background-image: url(images/layers-2x.png);
  background-size: 26px 26px;
}

[marcoaosilva]

commit a8e88d7