DELETE /cards/:card_id (delete a card)

[Roger-Heathcote]

Handler: /cards/delete.js
DELETE /cards/:card_id (auth) (same_user) -> { ok }
-- Note this is not as straightforward as it might look to implement
-- If a card is deleted from the deck we would then need to find all
-- the orderings that use that deck and delete the card_id from those
-- too! Stretch goal stuff maybe!
Note: we need to verify the user owns the deck.