[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.

PHPBB3-12409
Merge remote-tracking branch 'github-rxu/ticket/12409' into develop-ascraeus

* github-rxu/ticket/12409:
  [ticket/12409] Add acp_users.php core events to modify users preferences data