Make the album data an actual structured object

[fluffy-critter]

Instead of it being an ad-hoc dict-of-lists-of-dicts, it should be an actual data format that provides a JSON serializer/deserializer, which would make life so much easier with type-safety and also things like path normalization and so on, e.g. #69

Things will get slightly hairy with ad-hoc/extra attributes, though.