Clean up some unnecessary delivery_classes in tests.
Extract delivery_class from get_opted_in_bunches.
Use transport_type instead of delivery_class for subscription app.
Cleaner way to use transport_type instead of delivery_class.
Update app worker tests to use new-style conversation starting (for the most part) and fix reliance on tags and delivery class where necessary.
Rename conversation.start() to conversation.old_start() and update the world.
Rename new_start() to start().
Merge branch 'feature/issue-440-reduce-reliance-on-delivery_class' into develop