[ticket/12775] Move phpbb_convert_30_dbms_to_31 into the config file class

PHPBB3-12775
[ticket/12775] Remove useless includes of config.php

PHPBB3-12775
[ticket/12775] Fix doc blocks in the container builder

PHPBB3-12775
[ticket/12775] Set defined_vars as a property of config_php_file

PHPBB3-12775