Retina mode for map

[LGro]

On high density displays, the mapbox map embedded in lib/map.dart can look slightly blurry. Enable retina mode when appropriate by adding {r} to the urlTemplate along with handling retinaMode.isHighDensity and TileLayer.retinaMode accordingly. See https://docs.fleaflet.dev/layers/tile-layer#retina-mode for more details.