improvement only mode

[jef-pearlman]

I'd love a mode where it only includes maps where you already have met some threshold. The thresholds could be:

1. Any previous PB or no PB (how it currently works).
2. Only maps where you have a PB (improvements only).
3. Only maps where you already have bronze or better.
4. Only maps where you already have silver or better.
5. ... etc.

(You could also do things like a range of bronze-silver, or within x seconds of the target metal, but I think those are probably less useful.) This would be super-handy, particularly for things like TOTD where there are so many tracks and you might want to spend some time grinding tracks you've already made some progress on and not get tracks you've never seen.

I think this would be relatively straightforward to add given how you build your track list. And it would interact well with #37. Thanks!