Add "/disable" to disable a flashcard from the slash-menu.

When typing “/disable,” the options are “Disable descendant cards” and the already-checked options labelled as “Practice card forwards/backwards/bidirectionally,” which is an unintuitive choice when trying to disable cards. But a “/disable” slash-menu item should disable all flashcards on the rem, independent of its current direction and have an intuitive label, i.e. without requiring me to have to think about which way the cards are going.