Preset/stolperstein: Update address fields

- remove "addr:" field since small usage and discouraged
- add addr_string which is the thing used most often
- update addr_addr to only show if at least "memorial:addr:street" is present

data/fields/memorial/addr_addr.json

@@ -1,11 +1,14 @@
 {
     "type": "address",
-    "key": "addr",
+    "key": "memorial:addr",
     "keys": [
         "memorial:addr:city",
         "memorial:addr:street",
         "memorial:addr:postcode",
         "memorial:addr:housenumber"
     ],
-    "label": "Nearest building address"
+    "label": "Nearest Building Address",
+    "prerequisiteTag": {
+        "key": "memorial:addr:street"
+    }
 }

data/fields/memorial/addr_string.json

@@ -0,0 +1,5 @@
+{
+    "type": "text",
+    "key": "memorial:addr",
+    "label": "Nearest Building Address"
+}