Fix setting thread's seenPosts from page of posts.

We were inconsistently setting the thread's seenPosts property, depending on
what was being parsed (HTML or JSON) and whether the server gave us an updated
value.
Codify comment.
Sort posts when parsing JSON.
Fix unread posts and count.

Was setting the thread's seenPosts property before rendering the posts. Not
ideal but this fixes it for now. (Ideally the posts would render the same way
whenever the posts view asks. This way, posts render differently after the
first time the posts view asks.)