ConversationTree is now a plugin (not oldschool setting)

ConversationTree is now a plugin (not oldschool setting)

Conversation trees works pretty bad with the current layout, javascript
etc. So it's best if we separate it and work on it as a side-project. The
oldschool settings are currently being deprecated (or broken out like this).

I'll wait with removing User preferences for oldschool conversation tree,
since that might be reusable data. But I guess it will go in the near future.