Limit map to the maximum zoom of the current base layer

Fix maximum zoom levels for various layers and ensure that the
map can't be zoomed beyond the limit of the current base layer.

Fixes #1978

app/assets/javascripts/leaflet.map.js

@@ -61,6 +61,12 @@ L.OSM.Map = L.Map.extend({
       code: "G",
       name: I18n.t("javascripts.map.base.gps")
     });
+
+    this.on("layeradd", function (event) {
+      if (this.baseLayers.indexOf(event.layer) >= 0) {
+        this.setMaxZoom(event.layer.options.maxZoom);
+      }
+    });
   },
 
   updateLayers: function(layerParam) {