GET /cards/:card_id (get a particular card by it's card_id)

[Roger-Heathcote]

Handler: handlers/cards/get.js
GET /cards/:card_id (auth) (same_user OR public) -> {
current_position,
deck_length: integer,
card:
{ id, deck, front_text, front_image, back_text, back_image, important, color }
-- Probably not needed after all but if we do use it...
Note: we need to verify the user owns the deck or that deck is public.