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/integration_test.rb

@@ -45,15 +45,22 @@ class IntegrationTest < ActiveSupport::TestCase
     end
 
     def assert_app_is_down
-      response = app_response
-      debug_response_code(response, "502")
-      assert_equal "502", response.code
+      assert_app_error_code("502")
+    end
+
+    def assert_app_in_maintenance(message: nil)
+      assert_app_error_code("503", message: message)
     end
 
     def assert_app_not_found
+      assert_app_error_code("404")
+    end
+
+    def assert_app_error_code(code, message: nil)
       response = app_response
-      debug_response_code(response, "404")
-      assert_equal "404", response.code
+      debug_response_code(response, code)
+      assert_equal code, response.code
+      assert_match message, response.body.strip if message
     end
 
     def assert_app_is_up(version: nil, app: @app)