Warn when cards have unsaved changes

[iopq]

I have lost cards because I forgot to click "Save" since I immediately try to learn the cards after I enter them. Sometimes the very last card just gets lost because I didn't save it, and it's not obvious this happened since it's just not there

There's a way to prompt on unsaved changes, I'd like this to happen when cards are not saved

[iopq]

To clarify: I want to add a beforeunload event listener when any card has been changed without being saved

[lakesare]

I agree we should add it.

beforeunload should save the html of the card in localStorage; and, when this course is reopened, we should offer the Restore the flashcard; Dismiss the flashcard buttons.

The PR is welcome!