Add several new database commands.

  * `.post a b` -- return a Post with id `b` from `Board` with id `a`.
  * `.post_remove a b` -- remove that post, its reply tree, the attached files, and all references to both. Also works on threads.
  * `.board_remove a` -- remove all information about a board and all associated data.

Also, `.thread a b` will now raise KeyError if that tree is not actually a top-level thread.