Add borough to supported place classes (#1470)

Adds a new class value for the place layer, `borough`, and adds it to the OSM style with the same appearance as hamlets and suburbs.

layers/place/types.sql

@@ -4,7 +4,7 @@ $$
         PERFORM 'city_place'::regtype;
     EXCEPTION
         WHEN undefined_object THEN
-            CREATE TYPE city_place AS enum ('city', 'town', 'village', 'hamlet', 'suburb', 'quarter', 'neighbourhood', 'isolated_dwelling');
+            CREATE TYPE city_place AS enum ('city', 'town', 'village', 'hamlet', 'borough', 'suburb', 'quarter', 'neighbourhood', 'isolated_dwelling');
     END
 $$;