Switch basemap to ESRI WorldGrayCanvas (OSM tiles blocked)

OSM's tile server blocks apps without a valid identifying User-Agent
(contextily's default is a random UUID, which OSM rejects with an
'Access blocked' error tile). Switched to ESRI WorldGrayCanvas, a free
no-key light-gray basemap comparable to CartoDB Positron, and set a
proper User-Agent header (jetlag-maps/0.1) on contextily requests per
OSM tile usage policy.

copenhagen/config/styling.yaml

@@ -67,7 +67,7 @@ mask:
   color: white
 
 basemap:
-  provider: OpenStreetMap.Mapnik
+  provider: Esri.WorldGrayCanvas
   zoom: 14
 
 labels: