"Sort bookmarks by unread" setting maintains last post date ordering.

I think this is what people expect: threads sorted by last post date, just with
threads with new posts ahead of up-to-date threads.

* Add -[AwfulThread anyUnreadPosts] and keep it up to date when setting
  relevant properties.
* Relegate -[AwfulThread unreadPosts] back to getter, away from actual Core
  Data attribute.
* Reword setting copy.
* Fix reloading bookmarks table when changing "sort unread first" setting.