Revert "Start using acts_as_tree instead of manual threading"

This reverts commit 1bec7c5bae7783b6b6fe7c4268e617eade71d43d.

It turns out acts_as_tree doesn't work here because it doesn't rethread
the subtree in the way I need:

- The root articles have their children preloaded. This is good.
- The root articles' children do *not* have their children preloaded,
  even though the individual articles corresponding to them have been
  loaded. This is bad and leads to many select calls as we recursively
  render the conversation.