Adds support for custom upload URLs

Uses the baseurl and basedir variables returned from `wp_upload_dir` to generate the resized logo path. Fixes an issue where `job_manager_get_resized_image` will not be able to save the generated image, when the home URL is different from the domain of the uploads URL. 

Currently something like http://example.com/uploads/image.jpg works but  http://uploads.example.com/image.jpg does not because it is assumed that home_url is contained in the URL of the uploaded image and can be replaced by ABSPATH.

There are various ways to alter the uploads directory/url that `wp_upload_dir` supports (see [codex page](https://codex.wordpress.org/Function_Reference/wp_upload_dir))