add field for "drinking_water" tag on amenities

see #501 (comment)

Author: tyrasd

data/fields/drinking_water_available.json

@@ -0,0 +1,9 @@
+{
+    "key": "drinking_water",
+    "type": "check",
+    "label": "Drinking Water Available",
+    "terms": [
+        "drinkworthy",
+        "potable"
+    ]
+}

data/presets/tourism/camp_site.json

@@ -15,7 +15,7 @@
     "moreFields": [
         "backcountry",
         "dog",
-        "drinking_water",
+        "drinking_water_available",
         "email",
         "fax",
         "gnis/feature_id-US",

data/presets/tourism/wilderness_hut.json

@@ -13,7 +13,7 @@
     "moreFields": [
         "address",
         "capacity",
-        "drinking_water",
+        "drinking_water_available",
         "gnis/feature_id-US",
         "internet_access",
         "internet_access/fee",