Dialogues won't work properly over channels other than sms and ussd

Dialogue applications store each user's answers on their contact. For jsbox apps, we look up a contact based on the contact's address. At the moment, we need to explicitly set the delivery class for a jsbox conversation in order to figure out which type of contact address to use when looking up the contact, and default to `ussd` as the delivery class if none is given. Since `ussd` and `sms` delivery classes map to a contact's `msisdn`, dialogue apps will work for those two channel types. For other channels types (for e.g, twitter), this won't work, since we will still be looking up contacts by their msisdn.

The short way round would be to add a delivery class drop down to UI for dialogue apps.

The longer, better way round would be to change jsbox apps to figure out the type of contact address to look up based on the received message's address type. We will need praekelt/vumi#581 for this.