Provide a 'grade now' feature

[dae]

Useful to e.g. push cards forward after they've been reviewed externally, and because FSRS would prefer every rescheduling have a grade associated with it.

The scheduling code currently assumes that a card will only be answered when it's at the top of the scheduling queue. The queue is potentially tens of thousands of cards big, and it's not ideal if we have to search through the entire list on every card answer to locate the matched card and remove it from the queue (in queue/mod.rs).

Perhaps one way we could avoid unwanted extra processing is to refactor the card answering so that there is a from_queue flag. If false, the entire queue would be searched, and there would be no error generated if the card was not found in the queue (when e.g. the user has rated a card that's not due yet).

Presumably we should ignore suspended cards in such a mode, or throw an error when attempting to reschedule them?