Rename admin proposal notifications controller

To be consistent with all the other controllers dealing with hidden
content, we use the word "hidden" in the controller class.

config/routes/admin.rb

@@ -44,7 +44,7 @@ namespace :admin do
     end
   end
 
-  resources :proposal_notifications, only: :index do
+  resources :hidden_proposal_notifications, only: :index do
     member do
       put :restore
       put :confirm_hide