Fix error causing access tokens to be overwritten

* User access tokens were being overwritten when invitations were created
* Issue was the result of creating an instance of YammerUser class with current_user token information
* Remove YammerUser class
* Modify InviteeBuilder and SessionController to update user information as necessary
* Remove all instances of access tokens being set to anything other than response from omniauth
* Add translator class for data coming back from Yammer api
* Add validates uniqueness check for access_token to user model
* Add test and update specs where necessary
* Rename method responsible for merging existing guest accounts with new Yammer user accounts