Anonymous user support for the payment reference field

Depends on #2105793: Port Payment Reference Field.

The idea is to disallow anonymous users from using the field as long as the email module is not enabled. When it is, create an email field on payments of the payment_reference bundle that is accessible to anonymous users only. After submitting the payment form (and effectively making the payment) they will receive an e-mail with a coupon/code/token that can be used to claim the payment. A reference to the payment will be stored in their session, so when they return to the payment reference field widget, they can just as easily use the payment as authenticated users can. However, if the session is somehow terminated, they can use the coupon/code/token to claim the payment next time they view the field widget.

Thanks for the feedback!

This patch updates the payment reference payment type settings form (on /admin/config/services/payment/type/payment_reference) with additional configuration elements. It also extends the default configuration, and contains code for sending the email.

To do:

• Translate configuration.
• Create and store retrieval codes.
• Extend the form element with the option to accept a retrieval code.
• Unit test \Drupal\payment_reference\Plugin\Field\FieldType\PaymentReferenceItemList::defaultAccess()

Making the e-mail config translatable depends on #2154743: Support route parameters for *.config_translation.yml files' base_route_name properties.

@tstoeckler about fixing this:

Yes, you should write a custom mapper. You need to ask your plugins for their config names, and then add a route plugin and then add a mapper foreach route to the respective config names

Re-roll.

However, I am not sure we should add this feature. It's pretty complex, yet it may not suffice for many people as I can imagine not everyone will want to use the built-in method to generate reference codes, or so send the reference codes over e-mail in this particular way.

Instead of requiring payment before allowing visitors to create user accounts, visitors should perhaps be allowed to creata accounts without payment, and be redirected to a payment page directly after registration where making a payment grants them an additional user role.

If anonymous users are allowed to create nodes for money, the nodes can remain unpublished or have another type of flag set that is changed directly after the anonymous user makes a payment for that node.

And if site builders have a situation where the context cannot be interrupted, but they want anonymous users to make payments after all, maybe they should build their own solutions, are they would just be too specific for payment to support out of the box.

I'd like some feedback from everybody about this. What is most interesting, are technical suggestions to support this feature, that address the issues that I raised earlier in this comment. Thank you.

All the extra code is necessary to process payments for anonymous users in such a way that they can recover the payment in case the workflow is interrupted between making the payment and the system assigning it to something like an order. However, this is so specific that I am indeed inclined to just let this be, as it will not be sufficient for many people and the code does have to be maintained.