[D8] Introduce a new `file_url_transform_relative()` function

https://api.drupal.org/api/drupal/core%21includes%21file.inc/function/fi...

Transforms an absolute URL of a local file to a relative URL.

May be useful to prevent problems on multisite set-ups and prevent mixed content errors when using HTTPS + HTTP.

Parameters string $file_url: A file URL of a local file as generated by file_create_url().

Return value string If the file URL indeed pointed to a local file and was indeed absolute, then the transformed, relative URL to the local file. Otherwise: the original value of $file_url.