Half Leaflet’s zoom delta

With a map the size of ours, it always seemed like things were either
too zoomed out or too zoomed in.

web/static/map.js

@@ -19,6 +19,7 @@
         maxBounds: latLngBounds,
         maxBoundsViscosity: 1.0,
         zoomSnap: 0,
+        zoomDelta: 0.5,
         crs: L.CRS.Simple,
     });
     map.fitBounds(latLngBounds);