[FSRS] Separate Desired Retention for Young and Mature cards

[vaibhav]

Though not exactly what you requested for, Jarrett is developing a feature that will allow you to set different DRs for different decks having the same preset. Then, you can move your mature cards to a new deck and set a different DR for that deck.

@vaibhav

I wanted to implement a crude version of SSP-MMC on my decks, where I would use a low DR for cards with lower stability and a high DR for cards with higher stability. That would be impractical to implement without this feature.