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.
`channel_types` is basically the same as `go.vumitools.contacts.models.DELIVERY_CLASSES`. The UI needs it to know what channels people can choose to send over in the new 'send' state. At some point it will change to being only the relevant delivery classes. Happy to use `go.vumitools.contacts.models.DELIVERY_CLASSES` instead.

I'm calling it `channel_types`, since I'm guessing it will stick around when we replace delivery classes with channel types.