Placeholder for displayed price when using pricing phase

Show placeholder instead of price on order form when task is calculating pricing

As per the suggestion here, lookups from an external system via a task can be slow to generate pricing and show incorrect prices on the order form for a selected configuration. Instead, use a placeholder on the order form to show “calculating prices” or similar when the plan/configuration is changed rather than the old price.
Additionally, don’t allow the user to click “Next” until a price has been obtained - it is possible to select a “cheap” plan, then switch to an expensive one and click next before the pricing task has completed and the order summary shows the expensive plan but with the cheap price.

Example/Use case(s):
User selects plan → Pricing phase task (python) runs but takes a few seconds to complete → Form shows selected plan but with an incorrect cost → User clicks next → Order summary shows incorrect price for plan