FEATURE: Add ability to dismiss admin notices (#28916)

his is a new feature that lets admins dismiss notices from the dashboard. This helps with self-service in cases where a notice is "stuck", while we work on provisions to prevent "sticking" in the first place.

config/routes.rb

@@ -401,6 +401,8 @@ Discourse::Application.routes.draw do
           collection { put "/" => "about#update" }
         end
       end
+
+      resources :admin_notices, only: %i[destroy], constraints: AdminConstraint.new
     end # admin namespace
 
     get "email/unsubscribe/:key" => "email#unsubscribe", :as => "email_unsubscribe"