Store proper image url using default_storage

Simple modification to make the app store the URL of the image properly, based on the default_storage instead of MEDIA_URL. This is needed, for instance, if you store your files in remote platforms such as Amazon S3, so that the URL stored inside the contents must be absolute (e.g. "http://mybucket.s3.amazonaws.com/redactor/myfile.jpg" instead of "/media/myfile.jpg")