Fix detection of `unlisted` posts

[dato]

If followers_url is found in to, the post may still be unlisted if "https://www.w3.org/ns/activitystreams#Public" appears in cc. Hence this should be checked earlier.

[dato]

I will add a test and undraft if there's agreement the fix is good.

[mouse-reeve]

This seems like a good fix to me!