Add a form for choosing areas

Picking multiple areas at once definitely feels like a need, so let’s
make them checkboxes.

tests/app/main/views/test_broadcast.py

@@ -9,7 +9,6 @@ from tests.conftest import SERVICE_ONE_ID
     ('.preview_broadcast_areas', {}),
     ('.choose_broadcast_library', {}),
     ('.choose_broadcast_area', {'library_slug': 'countries'}),
-    ('.add_broadcast_area', {'area_slug': 'england'}),
     ('.remove_broadcast_area', {'area_slug': 'england'}),
     ('.preview_broadcast_message', {}),
 ))
@@ -76,23 +75,6 @@ def test_choose_broadcast_area_page(
     )
 
 
-def test_add_broadcast_area_page(
-    client_request,
-    service_one,
-):
-    service_one['permissions'] += ['broadcast']
-    client_request.get(
-        '.add_broadcast_area',
-        service_id=SERVICE_ONE_ID,
-        area_slug='england',
-        _expected_redirect=url_for(
-            '.preview_broadcast_areas',
-            service_id=SERVICE_ONE_ID,
-            _external=True,
-        ),
-    ),
-
-
 def test_remove_broadcast_area_page(
     client_request,
     service_one,