Rename ThreadItem to ContentThreadItem, then create a new ThreadItem
interface containing only the methods that we'll be able to implement
using only the persistently stored data (no parsing), then create a
DatabaseThreadItem. Do the same for CommentItem and HeadingItem.
ThreadItemSet gets a similar treatment, but it's basically only for
Phan's type checking. (This is sad.)
Change-Id: I1633049befe8ec169753b82eb876459af1f63fe8
