After POST data, check session for checkout step.
Keep step in session data up-to-date.
Handle exception when "order" isn't in session.
Merge remote-tracking branch 'upstream/master' into checkout_step_discovery
Forget checkout step when user POSTs to cart.
Merge remote-tracking branch 'upstream/master' into checkout_step_discovery
Forget checkout step every time the cart is shown.
Set session modified flag when remembering checkout step.
Forget checkout step only when creating new cart.
Show "Return to Checkout" if checkout in progress.
Merge