[JRO] Fix users' read issue with topic w/1 post

If there was a thread with 1 post a user's "read status" wouldn't be
saved - so they would click a thread, head out to the topic index and it
would still look unread.
[JRO] f u ios

sigh

How many different filenames can goddamn mobile safari request?
[#91] NullUser instead of User.new

![feels good man](http://www.vaultoflol.com/sites/default/files/156-feels-good-man-goat.jpg)

Close #91