I've updated all cases where we should have been using
`GetDisplayString()` anyway, but left the `ToString()` implementations
in place for safety. They should probably be removed in the future.
This matches the implementation we have for `BeatmapInfo` and
`BeatmapSetInfo`. All comparisons of `OnlineID` should be done directly
using them (ie. how it's done in `ScoreModelDownloader`).