Convert Payment Reference Field to general queue/reference solution

#1812594: Make the payment reference widget reusable paved the way for other modules to easily implement their own payment reference solution. However, they still need to do considerable work themselves, such as creating storage (queue) and a payment add page.

To accommodate this, we should put the queue logic in a separate module, and the reusable reference logic/UI elements as well. If we also add a token/coupon code/hash field to the queue, we can also support anonymous users (who cannot add payment references through the current system). The UI elements, such as the paymentreference form element can receive options to automatically use the queue for a certain context, so contexts will only have to use the form element to get a PID for a payment reference and not worry about storage or custom pages anymore.

Postponed to 7.x-2.x.