Pages should return 404 if an object cannot be found #239
Description
Permalinks such as [1] currently always return 200 status, even if the thread cannot be found [2].
If a link cannot be found, the page should return 404.
[1] https://lists.apache.org/thread/5j8l2vf9095dnd30kwcdhzg10rc32p3r
[2] https://lists.apache.org/thread/5j8l2vf9095dnd30kwcdhzgl0rc32p3r