[ticket/12408] Add quick setting for "default guest style" to ACP

Users often find it very confusing that there are different "default style" settings for logged-in users and guests.

Having to manually change the "user_style" of ANONYMOUS via the user settings seems counter-intuitive.

By adding a quick-select option for this would reduce a lot of this confusion.

Ticket: https://tracker.phpbb.com/browse/PHPBB3-12408
More info: https://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=44331&p=262887

PHPBB3-12408