[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/install/index.php

@@ -240,7 +240,7 @@ $sub = $request->variable('sub', '');
 // Set PHP error handler to ours
 set_error_handler(defined('PHPBB_MSG_HANDLER') ? PHPBB_MSG_HANDLER : 'msg_handler');
 
-$user = new \phpbb\user();
+$user = new \phpbb\user('\phpbb\datetime');
 $auth = new \phpbb\auth\auth();
 
 // Add own hook handler, if present. :o