Get delivery class from jsbox config for push triggers

At the moment, we get the delivery class off the conversation model: 
https://github.com/praekelt/vumi-go/blob/develop/go/apps/jsbox/definition.py#L25

The `Conversation` model's `delivery_class` attribute is no longer set now that we have routing table dispatchers, so we need to find the delivery class a different way.

The plan is to determine the delivery class from the jsbox conversation's config, since we rely on it on the toolkit side anyways. This will be easy enough to remove once we have proper address types.