resolve() should use relative_path to build media path

[Ryan Kelly]

The resolve() function searches for the media file using relative_path, but returns a path constructed using the original media_name. This can result in the returned media path using incorrect path separators on windows. I've modified my version to use this search loop:

    for mount_url, mount_path in get_mount_points():
        media_path = os.path.join(mount_path, relative_path)
        if os.path.lexists(media_path):
            return (
                posixpath.join(mount_url, media_name),
                media_path)