Improve the checkout API

#2689549: Implement the initial checkout structure has landed, and now we need to commit the panes.

The various issues in #2710973: [meta] Finish the checkout functionality require some API changes:
1) The CheckoutFlow getPreviousStepId() and getNextStepId() should be public so that panes can use them.
2) The "get checkout flow for order" logic from CheckoutController needs to move to a CheckoutOrderManager class so that it can be used by the checkout progress block
3) We should make sure processStepId() treats a bogus step id value the same as an empty one.