MAP-434 - Surface Address Picker In Data Mapping (#117)

* Add Address Picker to sources available

* Linting fix

* Further linting

hub/enrichment/sources/__init__.py

@@ -211,6 +211,11 @@
                 "value": "dates[0].polling_station.station.id",
                 "label": "Nearest Polling Station ID",
             },
+            {
+                "value": "address_picker",
+                "label": "Address Picker",
+                "description": "True if we need to show this user an address picker, as their postcode has more than one polling station",
+            },
         ],
     },
 }