Postal codes should not be required for some countries

[alaca]

User Story

As a donor from the country which doesn't have postal codes ( like Aruba ), I want to be able to make a donation without entering the postal code.

Details

Not all countries have postal codes, but the GiveWP donation forms currently require a postal code whenever the billing details are required.

Expected Behavior

A donor from the country which doesn't have postal codes should be able to make a donation without entering the postal code

Steps to Reproduce

1. Enable Offline Donation gateway
2. Enable Collect Billing Details setting for Offline Donation gateway
3. Go to the donation page and select country Aruba

Visuals

Acceptance Criteria

• When a country without postal codes is selected, the donation form should update to make optional the Zip/Postal Code field
• Donors from countries without postal codes should be able to make a donation without entering the postcode

[canny]

This issue has been linked to a Canny post: Postal codes should not be required for some countries 🎉