Add memorial=ghost_bike as string option to the memorial dropdown (#1302)

Author: mnalis

data/fields/memorial.json

@@ -7,6 +7,7 @@
             "blue_plaque": "Blue Plaque",
             "bust": "Bust",
             "cross": "Memorial Cross",
+            "ghost_bike": "Ghost Bike",
             "obelisk": "Obelisk",
             "plaque": "Commemorative Plaque",
             "sculpture": "Sculpture",