Show zones in campground map by default if requested by query parameters

This is to be able to link from the booking form, with a link under the
area preferences, and show the zones layer, that is what customers most
certainly want to see at that point.

web/static/map.js

@@ -205,4 +205,9 @@
         .addOverlay(zonesOverlay, zones.getAttribute('aria-label'))
         .addTo(map)
     ;
+
+    const params = new URLSearchParams(window.location.search);
+    if (params.has('zones')) {
+        map.addLayer(zonesOverlay);
+    }
 })();