Restore window-crop*-routes to support the thumbnail generation from ImageServing in MW

Restore window-crop-route and window-crop-fixed-route. Many of the existing extensions use the `ImageServing` interface which generates thumbnails of this form. We're adding it back in so we have a migration path to support the existing features and extensions that use this interface.

/cc @nmonterroso 
Done 66e7417d.