Add backend for the moderators

Add new routes for the proposal notifications edition and
abilities to let moderators edit it (mark as ignored, hide, etc.).

The notifications are not flaggable because they doesn't work like that,
but in a similar way. The moderator/administrator is in charge of hidding
them through the UI, so the normal users don't flag it as inappropriate.

New controller Moderation::ProposalNotification to manage the moderators
work.

config/routes/admin.rb

@@ -38,6 +38,13 @@ namespace :admin do
     get :summary, on: :collection
   end
 
+  resources :proposal_notifications, only: :index do
+    member do
+      put :restore
+      put :confirm_hide
+    end
+  end
+
   resources :budgets do
     member do
       put :calculate_winners