End to end test and post collision fix

There is a little bit of stuff in this set that might be overly opinionated, happy to modify or remove.

In the process of creating an end-to-end test for doing conversation stuff, I found #57, so this also fixes that, and a few other ways that folks could misbehave.  Nothing that I would call a security exploit, since you'd have to have the crumb value to post with a uid/name, but some defense in depth against garbage data that could be a pita to deal with otherwise.
I'm going to change this to 'next' instead of 'cb' for consistency