Keeping the training output folder structure intact bw workflow and board

[kegl]

At this time we move all predictions from <event>/submissions/<id>/training_output into <event>/predictions/<id>. In #449 we needed to create a symlink back to the original folder, and in #453 we also use the original folder to access the predictions through an on-the-fly property in the DB. In fact we probably don't need to move them at the first place. I open this issue to discuss this, whether we could just leave the folder structure as is.