Always include "confirm" state and transition to "complete" explicitly

Updates:

1. The order confirm step is always included *at the model level* (UIs could certainly hide it as needed)
2. Transitioning to order "complete" state must be done explicitly (e.g., not via "next")
3. Adds a new "confirm" page to the admin order flow

This should be a safer flow that prevents bugs like the one we saw where default credit cards would make an order immediately transition to complete once something was added.

I think there's more that could be improved about the checkout flow but this should be a solid improvement.