Fix status banner auth check to use current_scope

phx.gen.auth sets current_scope, not current_user. Use !! to
ensure boolean for HEEx template and register_and_log_in_user
in tests for proper auth session.

app/lib/firehose_web/controllers/blog_controller.ex

@@ -29,7 +29,7 @@ defmodule FirehoseWeb.BlogController do
       post: post,
       base_path: blog.base_path(),
       visibility: visibility,
-      authenticated: conn.assigns[:current_user] != nil
+      authenticated: !!(conn.assigns[:current_scope] && conn.assigns.current_scope.user)
     )
   end