Rewrite of card leaning / scheduling and migration to FSRS

[h16nning]

This is a big one.

The current implementation of learning and scheduling needs a complete rewrite because it is pretty messy and buggy.

Right now the SM2 algorithm is used. It is relatively old (I think 1987). Recently the free open-source algorithm Free Spaced Repetition Scheduler (FSRS) has been released. It is significantly more accurate than SM2 and based on scientific research. Ankis newest version offers a switch to this scheduler as well. There is already a JS implementation and another person is working on a Typescript version.

Additional explanation on the algorithm

I figured a rewrite would be a good opportunity to switch to FSRS.

[h16nning]

will use fsrsjs

[h16nning]

Closed with #41