Add OpenStreetMap Notes Layer (#458)

* use a local variable (best practices)

* Add OpenStreetMap notes as overlay.

This commit adds a new layer which renders OpenStreetMap notes in the
current view.

The layer can be activated via the "More" button in the layers sidebar.

Fixes #439

layers/config/tree.js

@@ -72,7 +72,8 @@ BR.confLayers.tree = {
             'Waymarked_Trails-Cycling',
             'Waymarked_Trails-Hiking',
             'Waymarked_Trails-MTB',
-            'mapillary-coverage-raster'
+            'mapillary-coverage-raster',
+            'osm-notes'
         ],
         'country': [
             'historic-place-contours',