Improve spaced repetition
Spaced repetition currently happens on fixed dates depending on when the reminder was created.
It might be best if we do it N days after the last reminder was viewed. That way, if someone skips a few dates, they'll still get the right number of repetitions.
With this approach, we would no longer keep the schedule as dates, and would instead keep:
- The number of days between repetitions, obtained from when it was created;
- The last number of days chosen for the last reminder scheduled.