Saleor 1515/customise order confirmation messages

[bmigirl]

I want to merge this change because it adds custom thank you subtitle depending on order status

Screenshots

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. The changes are tested.
4. The code is documented (docstrings, project documentation).
5. Changes are mentioned in the changelog.

Test Environment Config

API_URI=https://feature-order-confirmation.api.saleor.rocks/graphql/

[patrys]

Task linked: SALEOR-1515 Display different message on checkout "Thank you" page depending on confirmation