[ticket/12489] Description for Contact link in custom profile fields

[PHPBB3-12489](https://tracker.phpbb.com/browse/PHPBB3-12489)

Note that only %s wildcard can be used as per [/phpbb/profilefields/manager.php line 404 and 409](https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/phpbb/profilefields/manager.php#L404)
`%s` needs to be escaped as `%%s`, IIRC