Fix Duel model and get Duel#random_duel working

- Change `winning_photo:string` field to `winning_picture_id`
- Fix typo in `Duel` model (::base vs. ::Base)
- Define `Picture#random_ids`
- Have `Duel#random_duel` return a `Duel` instance
- `has_many :duels_pictures` -> `has_many :duel_pictures`