Maintenance mode

Adds support for maintenance mode to Kamal.

There are two new commands:
- `kamal app maintenance` - puts the app in maintenance mode
- `kamal app live` - puts the app back in live mode

In maintenance mode, the kamal proxy will respond to requests with a
503 status code. It will use an error page built into kamal proxy.

You can use your own error page by setting `error_pages_path` in the
configuration. This will copy any 4xx.html or 5xx.html files from that
page to a volume mounted into the proxy container.

test/integration/app_test.rb

@@ -48,9 +48,36 @@ class AppTest < IntegrationTest
     assert_match "App Host: vm1", exec_output
     assert_match /1 root      0:\d\d nginx/, exec_output
 
+    kamal :app, :maintenance
+    assert_app_in_maintenance
+
+    kamal :app, :live
+    assert_app_is_up
+
     kamal :app, :remove
 
     assert_app_not_found
     assert_app_directory_removed
   end
+
+  test "custom error pages" do
+    @app = "app_with_roles"
+
+    kamal :deploy
+    assert_app_is_up
+
+    kamal :app, :maintenance
+    assert_app_in_maintenance message: "Custom Maintenance Page"
+
+    kamal :app, :live
+    kamal :app, :maintenance, "--message", "\"Testing Maintence Mode\""
+    assert_app_in_maintenance message: "Custom Maintenance Page: Testing Maintence Mode"
+
+    second_version = update_app_rev
+
+    kamal :redeploy
+
+    kamal :app, :maintenance
+    assert_app_in_maintenance message: "Custom Maintenance Page"
+  end
 end