add "cai_scale" as optional field for hiking route relations in Italy

closes #293

data/fields/cai_scale.json

@@ -0,0 +1,22 @@
+{
+    "key": "cai_scale",
+    "type": "combo",
+    "label": "Hiking Route Difficulty (Italian Alpine Club)",
+    "placeholder": "T, E, EE, …",
+    "strings": {
+        "options": {
+            "T": "T: Touristic",
+            "E": "E: For hikers",
+            "EE": "EE: For expert hikers",
+            "EEA": "EEA: For expert hikers with equipment",
+            "EAI": "EAI: Hiking in snowy conditions"
+        }
+    },
+    "snake_case": false,
+    "caseSensitive": true,
+    "autoSuggestions" : false,
+    "customValues" : false,
+    "locationSet": {
+        "include": [ "IT" ]
+    }
+}