The previous commit changed our response for the root domain on a
self-hosted Zulip server containing no realms to be a 404.
This made the HTTP headers verification done here less robust than
would be ideal, in that one could imagine a bug that makes every URL
404 passing that test.
So we instead change the check to verify the /server_settings API,
which doesn't have that problem.
To provide a smoother experience of accessing a web public stream,
we don't ask user to login unless user directly requests a
`/login` URL.
Fixes#21690.
success-http-headers-bionic.txt and success-http-headers-focal.txt
differ only in the nginx version so this substitution will allow
us to have single file for both of them. Also this change helps
to avoid CI failure if Nginx version is updated in the OS.
Tim Abbott
Aman Agrawal
Anders Kaseorg
arpit551