[Backport 2.2]Add VAT number to email source variables

[JeroenVanLeusden]

Description

Backport of #10996

In some countries (the netherlands) it's obligated to display your VAT Number in order/invoice emails/pdfs. This field isn't available in the configVariables and cannot be shown using a config path.

Fixed Issues (if relevant)

1. None that I could find.

Manual testing scenarios

1. Try to add your VAT Number using {{config path="general/store_information/merchant_vat_number"}}.
2. Empty value is shown.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)