Make the conversion of order IDs to numbers toggled in the gateway settings

As of 2.3.0 we now update payments at Affirm so the "Order ID" column there shows the order number after the order has been placed in Drupal Commerce. (This allows us to forego early generation of order numbers.) However, it's possible depending on the use case that a site would prefer to retain the actual serial numeric ID as the "Order ID" in Affirm.

Let's add a setting to the payment gateway between the display name toggle and the API request / response logging toggle labeled, "After an order is placed in Drupal, update its payment in Affirm to show the order number as the order ID." Its help text should read, "This is helpful for sites that use the order number in the admin interface." It should default to TRUE.