Add confirmation screen when handling reports (#22375)

* Add confirmation screen on moderation actions

* Add flash notice when a report has been processed

* Refactor tests

* Add tests

config/routes.rb

@@ -310,7 +310,11 @@ Rails.application.routes.draw do
     end
 
     resources :reports, only: [:index, :show] do
-      resources :actions, only: [:create], controller: 'reports/actions'
+      resources :actions, only: [:create], controller: 'reports/actions' do
+        collection do
+          post :preview
+        end
+      end
 
       member do
         post :assign_to_self