Get dialogue app's endpoints to show up

At the moment, we figure out which endpoints to use in a dialogue app for the vumi app side, but not the UI side. We need to get the dialogue app's conversation definition to know how to do that.
Since this method obtains and sets the conversation's extra endpoints, we are relying on the fact that all values are unicode because of the json deserializing. If it were to somehow give a non-unicode value as an endpoint, an exception would get raised. Not sure if its something we need to worry about, problem not, but it all seems quite implicit at the moment.