Load thread if requested thread isn't loaded yet.

Reapplies the changes in https://github.com/edx-solutions/xblock-discussion/pull/30 which were accidentally omitted.
@Kelketek read comment below first...


Now, once you've read it, this could be achieved like 
    @retrieveSingleThread.done(
        (data, textStatus, xhr) => 
            @thread = new Thread(data['content'])
            @renderThreadView()
    )

Which is a bit cleaner IMHO.