Add possibility to create and add a 'security.txt' file for forums

[supermathie (Michael Brown)]

I would suggest using a Permalink here for this, e.g.:

○ → curl -i https://meta.discourse.org/.well-known/security.txt
HTTP/2 301 
content-type: text/html; charset=utf-8
location: https://hackerone.com/discourse

(I’ll probably remove it shortly as it’s not really the intended destination, but this means you can put this file anywhere)