add counties page

What was previously ward -> local authority is now a ward -> local
authority -> county. County only covers rural counties and not
metropolitan boroughs and other unitary authorities. Previously, there
was a page full of local authorities (unitary authorities and
districts), and each one of those would have a list of electoral wards.
However, now there are counties that contain a list of districts - so
this needs a new page - a checkbox for "select the county" and then a
list of links to district pages.

If you want to select multiple districts, you'll need to go into each
one of those sub-sections in turn and click select all.

Needed to tweak the query to retrieve the list of areas in a list for a
library. Previously, it just returned anything at top level (ie: didn't
have a parent). However, rural districts now have parents (the rural
counties themselves). So the query now returns "everything that isn't a
leaf node", or in more specific terms, everything that has at least
other row referring to it as a parent. So no electoral wards, since
they dont have any children, but yes to districts and counties.

app/main/views/broadcast.py

@@ -160,8 +160,10 @@ def choose_broadcast_sub_area(service_id, broadcast_message_id, library_slug, ar
     )
     area = BroadcastMessage.libraries.get_areas(area_slug)[0]
 
+    is_county = any(sub_area.sub_areas for sub_area in area.sub_areas)
+
     form = BroadcastAreaFormWithSelectAll.from_library(
-        area.sub_areas,
+        [] if is_county else area.sub_areas,
         select_all_choice=(area.id, f'All of {area.name}'),
     )
     if form.validate_on_submit():
@@ -171,6 +173,20 @@ def choose_broadcast_sub_area(service_id, broadcast_message_id, library_slug, ar
             service_id=current_service.id,
             broadcast_message_id=broadcast_message.id,
         ))
+
+    if is_county:
+        # area = county. sub_areas = districts. they have wards, so link to individual district pages
+        return render_template(
+            'views/broadcast/counties.html',
+            form=form,
+            search_form=SearchByNameForm(),
+            show_search_form=(len(area.sub_areas) > 7),
+            library_slug=library_slug,
+            page_title=f'Choose an area of {area.name}',
+            broadcast_message=broadcast_message,
+            county=area,
+        )
+
     return render_template(
         'views/broadcast/sub-areas.html',
         form=form,