Extend payments page in WordPress admin dashboard with billing/shipping country filter

[remcotolsma]

We have considered moving all payment information to custom tables, but I think this is too big a step for now. In order to give users guidelines for the new VAT rules, I propose to store billing and shipping country code in the payment meta for now.

• _pronamic_payment_billing_country_code
• _pronamic_payment_shipping_country_code

The existing thresholds for distance sales of goods within the EU will be abolished and replaced by a new EU-wide threshold of EUR 10 000. Below this EUR 10 000 threshold, the supplies of TBE (telecommunications, broadcasting and electronic) services and distance sales of goods within the EU may remain subject to VAT in the Member State where the taxable person is established.

Source: https://ec.europa.eu/taxation_customs/business/vat/vat-e-commerce_en

Internal Basecamp to-do: https://basecamp.com/1810084/projects/10966871/todos/418493236

[rvdsteege]

@remcotolsma I'd like to propose to include stats per country (and option to filter payments by country) in a Reporting add-on project, instead of adding a single filter like this to the existing payments list page.