Fixed up de-duplication issues

This finishes up the de-duplication of adding users to a conversation as well as adding a conversation to a user's conversation list.  It should also restore functionality to many things that are currently broken on master.

Conversation.js is getting a bit messy and bloated, perhaps a refactoring should be considered in a future PR.
Do we not need to specify 'db.models' anymore? I totally trust that your code works, but I'm just curious for my own knowledge why this is!