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

admin_activate_user.php notification type is assigning usernames to email
template variables 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.

PHPBB3-13223
Merge branch 'ticket/13223' into prep-release-3.1.0

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