Make cards archivable (+ realtime)

Cards:

* https://trello.com/c/LleqDRDu/9-archive-a-card-initially-there-won-t-be-a-way-of-unarchiving-or-seeing-archived-cards
* https://trello.com/c/0OPMOQBM/26-archive-cards-in-realtime

@exalted, when you have some time :wink: