Postal Code field is always labeled as such, even for United States

Many countries have subtle differences in regards to fields needed for an address. However, the enrollment form is currently static.

Let's consider using a module such as https://www.npmjs.com/package/postal-address-field-names for this.

//Reported by @agafgfafa https://github.com/EclipseFdn/react-eclipsefdn-members/issues/191#issuecomment-882731077