Escape element types

Element type slugs with spaces are not allowed by Arkindex. We need to escape them if there are any.

• https://gitlab.teklia.com/workers/yolo/-/blob/main/worker_yolo/segmenter.py?ref_type=heads#L20
• https://gitlab.teklia.com/workers/yolo/-/blob/main/worker_yolo/segmenter.py?ref_type=heads#L51