Use already-transformed polygons wherever possible

Transforming full resolution polygons to linear coordinates is somewhat
expensive. We can make things run faster by:
- looking at `simple_polygons` which have fewer points and are therefore
  faster to transform.
- reusing polygons that have already been transformed where possible

app/models/broadcast_message.py

@@ -88,7 +88,7 @@ class BroadcastMessage(JSONModel):
             broadcast_message_id=broadcast_message_id,
         ))
 
-    @property
+    @cached_property
     def areas(self):
         if 'ids' in self._dict['areas']:
             library_areas = self.get_areas(self.area_ids)