Make open purchase order limit optional

Problem/Motivation

We have a requirement to allow an arbitrary number of open purchase orders. We use a back-end system for accounting and accounting personnel do not update purchase orders in Drupal to mark them paid. Setting the PO limit to even a relatively high number could lead to eventual problems at checkout that our managers would not easily diagnose.

Since open purchase order limits is a business rule, it seems reasonable to accommodate a business case where there is no need for a limit.

Proposed resolution

Add a checkbox "Add a limit to open purchase orders", which, when checked, reveals the limit field. In authorizePayment() check if the box is checked before checking if the limit is exceeded.

Remaining tasks

Create patch.

User interface changes

A checkbox on the gateway configuration.

Data model changes

Need a schema update.