[ticket/12961] Add link in anti-spam ACP page which links directly to captchas in titania

Since many of the CAPTCHAs that will be shipping with 3.1 are broken, we should add a (big) link, which goes directly to the anti-spam category/tag in the extensions database on phpbb.com.

Additionally, we might want to include a link to the knowledge-base. Or this post: https://www.phpbb.com/community/viewtopic.php?f=46&t=2122696

https://tracker.phpbb.com/browse/PHPBB3-12961
But we want to indicate that we are going to a specific page within the ext database (where all the anti-spam ext are located).

If you think the text of the link shouldn't be that detailed, that's fine with me.