[ticket/13223] Fix USERNAME email template variable in notification

admin_activate_user.php notification type is assigning username to email
template using user_loader->get_username() in 'no_profile' mode,
which in its turn calls get_username_string() function in 'no_profile' mode.
This causes HTML markup in emails for username colour.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13223">PHPBB3-13223</a>.