Define the (delivery_class, addr_type) mappings in models.py for contacts
Show delivery classes on dialogue edit screen
Use the configured poll delivery class as the jsbox config's delivery class in PollConfigResource
Change client side dialogue view and model to handle rendering and saving the delivery class
Add the delivery class as part of the context given to the dialogue edit screen's template
Add labels for delivery classes
Merge branch 'feature/issue-911-dialogues-wont-work-properly-over-channels-other-than-sms-and-ussd' into develop