404 page should allow login

**_1 Upvote**_ If a forum is set to members only and you click on a direct link to a post (which may have been posted in IRC, Discord, Email, etc) the user will get a 404 error page instead of a login page.

I think it should redirect to a login page instead.

Start with the direct-link handling for posts in members-only forums and trace why unauthenticated users receive a 404 instead of the login flow. Done means a direct post link opens the login page when authentication is required, rather than showing a 404.

Written by the indexing model from the issue text.