Add Sidekiq, configure admin access to Web UI

config/routes.rb

@@ -1,3 +1,5 @@
+require 'sidekiq/web'
+
 Rails.application.routes.draw do
   resources :donations
   devise_for :users
@@ -23,6 +25,10 @@ Rails.application.routes.draw do
     resources :donations
   end
 
+  authenticate :user, ->(user) { user.is_admin? } do
+    mount Sidekiq::Web => '/sidekiq'
+  end
+
   # Letter Opener (open "sent" emails in dev and staging)
   if Rails.env.match(/staging|development/)
     mount LetterOpenerWeb::Engine, at: "letter_opener"