Let users select electoral wards of local authorities

If a library has groups, we should show a link instead of selecting the
group directly.

Then we can give the user the choice of selecting the whole of that
group, or specific areas within the group.

For now the only libraries we have with groups are local authorities,
which group electoral wards.

app/main/views/broadcast.py

@@ -12,6 +12,7 @@ from app import current_service
 from app.main import main
 from app.main.forms import (
     BroadcastAreaForm,
+    BroadcastAreaFormWithSelectAll,
     ChooseBroadcastDurationForm,
     SearchByNameForm,
 )
@@ -118,6 +119,16 @@ def choose_broadcast_area(service_id, broadcast_message_id, library_slug):
         service_id=current_service.id,
     )
     library = BroadcastMessage.libraries.get(library_slug)
+
+    if library.is_group:
+        return render_template(
+            'views/broadcast/areas-with-sub-areas.html',
+            search_form=SearchByNameForm(),
+            show_search_form=(len(library) > 7),
+            library=library,
+            broadcast_message=broadcast_message,
+        )
+
     form = BroadcastAreaForm.from_library(library)
     if form.validate_on_submit():
         broadcast_message.add_areas(*form.areas.data)
@@ -136,6 +147,41 @@ def choose_broadcast_area(service_id, broadcast_message_id, library_slug):
     )
 
 
+@main.route(
+    '/services/<uuid:service_id>/broadcast/<uuid:broadcast_message_id>/libraries/<library_slug>/<area_slug>',
+    methods=['GET', 'POST'],
+)
+@user_has_permissions('send_messages')
+@service_has_permission('broadcast')
+def choose_broadcast_sub_area(service_id, broadcast_message_id, library_slug, area_slug):
+    broadcast_message = BroadcastMessage.from_id(
+        broadcast_message_id,
+        service_id=current_service.id,
+    )
+    area = BroadcastMessage.libraries.get_areas(area_slug)[0]
+
+    form = BroadcastAreaFormWithSelectAll.from_library(
+        area.sub_areas,
+        select_all_choice=(area.id, f'All of {area.name}'),
+    )
+    if form.validate_on_submit():
+        broadcast_message.add_areas(*form.selected_areas)
+        return redirect(url_for(
+            '.preview_broadcast_areas',
+            service_id=current_service.id,
+            broadcast_message_id=broadcast_message.id,
+        ))
+    return render_template(
+        'views/broadcast/sub-areas.html',
+        form=form,
+        search_form=SearchByNameForm(),
+        show_search_form=(len(form.areas.choices) > 7),
+        library_slug=library_slug,
+        page_title=f'Choose an area of {area.name}',
+        broadcast_message=broadcast_message,
+    )
+
+
 @main.route('/services/<uuid:service_id>/broadcast/<uuid:broadcast_message_id>/remove/<area_slug>')
 @user_has_permissions('send_messages')
 @service_has_permission('broadcast')