Add function method to aggregate areas

This is standalone for now - we'll plumb it in later.

app/broadcast_areas/utils.py

@@ -0,0 +1,10 @@
+def aggregate_areas(areas):
+    areas = _aggregate_wards_by_local_authority(areas)
+    return areas
+
+
+def _aggregate_wards_by_local_authority(areas):
+    return {
+        area.parent if area.id.startswith('wd20-')
+        else area for area in areas
+    }