[ticket/12932] Add method for getting the datetime class and allow overriding

An extension can now override the datetime class by overwriting the
datetime.class parameter in their own yml file.

PHPBB3-12932

phpBB/phpbb/db/migration/data/v310/soft_delete_mod_convert.php

@@ -121,7 +121,7 @@ class soft_delete_mod_convert extends \phpbb\db\migration\migration
 			new \phpbb\auth\auth(),
 			$this->config,
 			$this->db,
-			new \phpbb\user(),
+			new \phpbb\user('\phpbb\datetime'),
 			$this->phpbb_root_path,
 			$this->php_ext,
 			$this->table_prefix . 'forums',