Add sidekiq admin rack application (#4158)

* Add sidekiq-admin rack app with basic auth, pro ui, and scheduler ui
* Clearly specify port and server to use for rackup
* Ensure sidekiq-admin is only started manually

sidekiq-admin.ru

@@ -0,0 +1,19 @@
+# Run sidekiq-admin: `SIDEKIQ_USERNAME=foo SIDEKIQ_PASSWORD=bar REDIS_URL=redis://some.elasticache.aws.url bundle exec rackup sidekiq-admin.ru -p 3000 -o 0.0.0.0`
+
+require 'sidekiq'
+require 'sidekiq-pro'
+require 'sidekiq-scheduler/web'
+require 'sidekiq/pro/web'
+
+Sidekiq.configure_client do |config|
+  config.redis = { :size => 1 }
+end
+
+map '/sidekiq' do
+  use Rack::Auth::Basic, "Protected Area" do |username, password|
+    Rack::Utils.secure_compare(::Digest::SHA256.hexdigest(username), ::Digest::SHA256.hexdigest(ENV["SIDEKIQ_USERNAME"])) &
+      Rack::Utils.secure_compare(::Digest::SHA256.hexdigest(password), ::Digest::SHA256.hexdigest(ENV["SIDEKIQ_PASSWORD"]))
+  end
+
+  run Sidekiq::Web
+end