Redirect to ‘view’ page after cancelling

This is better confirmation of your action than the dashboard, because
everything will stay the same except the thing you’ve just done.

app/main/views/broadcast.py

@@ -152,6 +152,7 @@ def cancel_broadcast_message(service_id, broadcast_message_id):
         service_id=current_service.id,
     ).cancel_broadcast()
     return redirect(url_for(
-        '.broadcast_dashboard',
+        '.view_broadcast_message',
         service_id=current_service.id,
+        broadcast_message_id=broadcast_message_id,
     ))

tests/app/main/views/test_broadcast.py

@@ -382,8 +382,9 @@ def test_cancel_broadcast(
         service_id=SERVICE_ONE_ID,
         broadcast_message_id=fake_uuid,
         _expected_redirect=url_for(
-            '.broadcast_dashboard',
+            '.view_broadcast_message',
             service_id=SERVICE_ONE_ID,
+            broadcast_message_id=fake_uuid,
             _external=True,
         ),
     ),