MDL-71135 course: add legacy_format_renderer support
Until Moodle 4.0, renderer.php file was optional (although highly recommended) for course formats. From Moodle 4.0 onwards, renderer is required to support the new course editor implementation. The legacy_format_renderer class has been created for backward compatibility, to avoid some errors with course formats (such as social) without the renderer file. Apart from that, course_format->get_renderer() method has been reviewed to use this legacy_format_renderer when no renderer.php file is found.