Themes: add compile time check for 'sameAs'-filters that the referenced layer actually exists

assets/layers/bike_parking/bike_parking.json

@@ -683,6 +683,10 @@
         "en": "Who maintains this bicycle parking?",
         "nl": "Wie beheert deze fietsenstalling?"
       },
+      "render": {
+        "en": "This bicycle parking is maintained by {operator}",
+        "nl": "Deze fietsenstalling wordt beheerd door {operator}"
+      },
       "freeform": {
         "key": "operator"
       }