Support custom broadcasts (without area IDs)

Custom broadcasts created directly via the API app won't have area
IDs since [1], where we started to distinguish between "names" (all
broadcasts have these) and IDs (for broadcasts created in this app).
We forgot to propagate the distinction into the code here.

This code fixes the bug for all broadcasts created after [1]. Any
custom broadcasts created before [1] will have their "ids" field set
instead of "names" so we won't show anything for them. This seems
reasonable as we don't support custom broadcasts yet.

[1]: 023a06d5fb

app/models/broadcast_message.py

@@ -89,10 +89,9 @@ class BroadcastMessage(JSONModel):
 
     @property
     def areas(self):
-        polygons = self._dict['areas']['simple_polygons']
-        library_areas = self.get_areas(self.area_ids)
+        if 'ids' in self._dict['areas']:
+            library_areas = self.get_areas(self.area_ids)
 
-        if library_areas:
             if len(library_areas) != len(self.area_ids):
                 raise RuntimeError(
                     f'BroadcastMessage has {len(self.area_ids)} areas '
@@ -101,8 +100,8 @@ class BroadcastMessage(JSONModel):
             return library_areas
 
         return CustomBroadcastAreas(
-            area_ids=self.area_ids,
-            polygons=polygons,
+            names=self._dict['areas']['names'],
+            polygons=self._dict['areas']['simple_polygons'],
         )
 
     @property