Issue #2031353 by pwolanin, dawehner, katbailey: Fixed URLgenerator broken for Drupal installed in a subdirectory - doesn't have a way to get a Drupal path.

Issue #1888424 by katbailey, steveoliver, twistor, beejeebus, effulgentsia: Change notice: Make Drupal's URL generation logic available to HttpKernel, and minimize code repetition/divergence.