Update labels.json parsing during training

Structure of this file has been simplified, we no longer need https://gitlab.teklia.com/atr/dan/-/blob/fee9392a6047bbe4766a34e4a361c170fdcf7035/dan/ocr/manager/dataset.py#L98-L100

changed milestone to %DAN-P5: Improve training

added P3 Quick Win labels

assigned to @mblanco

mentioned in merge request !269 (merged)

closed with merge request !269 (merged)