add "unleashed" as value for dog tag field

and limit offered values of `dog` field on dog-specific features to just =leashed`/`unleashed`: `no` does generally not make sense on these, and `yes`/`designated` is already implied by the primary tag.

data/fields/dog.json

@@ -6,6 +6,7 @@
         "options": {
             "yes": "Allowed",
             "leashed": "Leashed Only",
+            "unleashed": "Leashes Not Required",
             "no": "Not Allowed"
         }
     },