discogs: Set medium_total in TrackInfo

[sampsyo]

The Discogs plugin currently doesn't set medium_total, which can lead to incorrect tracktotal fields when per_disc_numbering is on.

Overall, the situation with medium grouping leaves something to be desired in the current implementation. We're duplicating a lot of information across the TrackInfo.index, TrackInfo.medium_index, TrackInfo.medium, and TrackInfo.medium_total fields along with the ordering in the AlbumInfo.tracks list and the AlbumInfo.mediums field.

Maybe we should overhaul these so that the information is stored in one place—for example, by allowing tracks to be a list of lists, or using a medium_lengths field to indicate the size of each medium. The goal here would be to minimize the headache for the data source author necessary to get the numbering correct.

[diego-plan9]

Added medium_total to TrackInfo during imports from the discogs plugin on #2222 (fb398c5). I'm leaving the issue open as the overhauling of the fields did not take place (and seems like it would indeed be a good idea, organization-wise).

[sampsyo]

Great; thank you! If it's OK with you, I'll close this issue since we no longer have a bug here. The refactoring would still be good to consider, but we can leave that to a separate effort.