Since we want to make more things auto-edits, it seems like it would make sense to make reordering mediums an auto-edit, like we did for reorder cover art.

One thing in particular this would help with is when someone wants to merge a release entered as "disc 2" into a release which already has discs 1 and 2. The disc first has to be moved to position 2 in order to merge corresponding mediums. We should really fix merging too, but that's far more difficult.