Realtime cards

TODO:

* [x] Add new cards in realtime
* [x] Update cards position in realtime
As for these `Pusher.trigger(...)`, for now I think we can keep them inside our controllers. I see this as a “web interaction” and thus it can be kept right here.

I presume that in the future we will need something more articulated than this, but I am pretty sure that we have to wait until then.