* Enable state-dependent custom scheduling data
* Next(Card)States -> SchedulingStates
The fact that `current` was included in `next` always bothered me,
and custom data is part of the card state, so that was a bit confusing
too.
* Store custom_data in SchedulingState
* Make custom_data optional when answering
Avoids having to send it 4 extra times to the frontend, and avoids the
legacy answerCard() API clobbering the stored data.
Co-authored-by: Damien Elmes <gpg@ankiweb.net>
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