Queue classifications

Closes #48 
The UserEnqueuedSubject instance is used to serve certain subjects via the subjects_controller#index route, as such I don't really think it's a denormalized view of the classifications model. 

Instead of creating an enqueued classification that's waiting on the annotations from the user another implementation could exist without any reference to a classifications instance and just take the subject and User id's from the classifications_controller#create action dequeue that way. Enqueueing could also happen via a subjects_controller#enqueue_gold_standard (or other) end point.

We can leave it like this for now but I think we're coupling models and conflating use-cases here.