Don't crash if a new thread's link can't be found.

Presumably this indicates some kind of error, but I can't be arsed to find out.
Break retain cycle when quoting a post or marking a thread read up to a post.

We were seeing some crashes from UIKit that stemmed from presenting the replyViewController after retrieving a quote. I don't know if this is the fix, as the error message is just vague enough, but it's a fix that's needed anyway.