Calculate score based on fields actually shown on cards

[tsudoko]

Right now the score is calculated based on review data from all cards from a matching note, regardless of whether the cards in question actually use the fields selected by the user, e.g. if the user selects "kanji" and the note has 2 cards, only one of which uses the "kanji" field, the score will be calculated from both cards anyway.

Calculating the score from actual cards is going to be slower, but it might still be in an acceptable range, we'd need to implement it to know for sure.

(To have full certainty if a given field is used in the card, we'd need to expand the template separately for each card, since some fields could be hidden with {{#field}}{{/field}}. Going that far would be even slower and likely not worth the effort.)