Support aggregating local authority clusters

This applies some heuristics to try and keep the overall list of
areas short when many are selected in the same wider area.

Currently we only have relationship information between upper and
lower tier local authorities, so we can't / won't aggregate up to
Greater London (it's own special thing) or whole countries.

tests/app/broadcast_areas/custom_polygons.py

@@ -37,3 +37,19 @@ CHELTENHAM = [
     [51.9328, -2.0221],
     [51.9324, -2.1265],
 ]
+
+CHELTENHAM_AND_GLOUCESTER = [
+    [51.8820, -2.2920],
+    [51.8234, -2.2570],
+    [51.8883, -2.0262],
+    [51.9425, -2.0771],
+    [51.8820, -2.2920],
+]
+
+SEVERN_ESTUARY = [
+    [51.5719, -2.9388],
+    [51.4180, -2.7259],
+    [51.8493, -2.1958],
+    [51.8883, -2.3016],
+    [51.5719, -2.9388],
+]