Changes since beta 1:
- Don't show multiple cancel buttons
- Fix cancel signal being ignored if sent quickly
- Prohibit weight generation when reps < 1000
- Allow cards with no learning history when not training
- UI tweaks
- Support retrievability calculation for (re)learning cards
- Fix issues with searching and sorting on retrievability
- Cap optimal retention simulation to 10 years
- Expose deck-then-random option in UI
- Fix entire collection being used to calculate weights