Extract documents action from Answer controller

This way we have a controller just to manage
Poll::Question::Answer related documents in the
same way we have for videos and images.

config/routes/admin.rb

@@ -173,7 +173,7 @@ namespace :admin do
       resources :answers, only: [], controller: "questions/answers" do
         resources :images, controller: "questions/answers/images"
         resources :videos, controller: "questions/answers/videos"
-        get :documents, to: "questions/answers#documents"
+        resources :documents, only: [:index, :create], controller: "questions/answers/documents"
       end
       post "/answers/order_answers", to: "questions/answers#order_answers"
     end