Add a review mode: hide all flashcards' backward in the document page

In this mode, all the backward of flashcards are hided in the document view, not queue view. And as mouse cursor move to something hided, this thing display immediately. As the gif show:
v2-6a78e13ea4b2a0a3ad7479f9e3189977_hd

A CSS solution:
RemNote Library (hannesfrank.github.io)