Automatically configure dialogue endpoints

Once #1042 lands, we will need a way of automatically determining which endpoints to use based on the chosen channel types.
Yup, this is an intentional restriction to avoid confusing people. I'm not quite sure if its a good idea, I guess it depends if there is a common use case for dialogues with multiple endpoints that have the same delivery class. @badlard and @SteveBarnett seemed to prefer this approach, and apparently the project this feature is for won't need multiple endpoints with the same delivery class. I'm still a bit wary about this decision though.