We can no longer change IDs so easily, because they're stored in the
permalink database, so remove this mechanism to make sure it's not
accidentally used in the future.
Change-Id: I392ee1f49c48fc2f23d05e9a37c643438b4f2b9a
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
