[ticket/12409] Add acp_users.php core events to modify users preferences data

There're events to modify UCP user preferences in ucp_prefs.php, so it makes sense to add related events to acp_users.php to control new user's prefs via ACP.

<a href="https://tracker.phpbb.com/browse/PHPBB3-12409">PHPBB3-12409</a>.