add buildin 'free' filter

assets/layers/campsite/campsite.json

@@ -286,35 +286,7 @@
     "mastodon"
   ],
   "filter": [
-    {
+    "free",
-      "id": "fee_filter",
-      "options": [
-        {
-          "question": {
-            "en": "Fee",
-            "ca": "Taxa",
-            "cs": "Poplatek",
-            "cy": "Ffi",
-            "de": "Gebühr",
-            "it": "A pagamento"
-          }
-        },
-        {
-          "question": {
-            "en": "free of charge",
-            "ca": "Gratuït",
-            "cs": "zdarma",
-            "de": "kostenlos",
-            "it": "gratuito"
-          },
-          "osmTags": {
-            "and": [
-              "fee=no"
-            ]
-          }
-        }
-      ]
-    },
     {
       "id": "capacity_persons_filter",
       "options": [

assets/layers/caravansites/caravansites.json

@@ -935,5 +935,8 @@
     "questions",
     "reviews"
   ],
+  "filter": [
+    "free"
+  ],
   "allowMove": true
 }