Cannot start thumbnail generation processes

1. Create a process anywhere.
2. Select any element in the filtering page.
3. Open Advanced settings and click on Generate thumbnails.
4. Click on Run process.
5. Watch the 400 error!

This is caused by worker activities now being enabled by default: the frontend just sends {"thumbnails": true, "worker_activity": true}, but worker activities are forbidden on thumbnail generation. The frontend should only send worker_activity: true when not generating thumbnails.