Setting to make VAT number required or optional

[AndreiMaz]

Let's add this setting to require VAT numbers to be entered. It could be very useful for B2B shops.

Add this setting to Admin area > Tax settings page > VAT block. It'll be a new child "'VAT number' required" setting displayed below the "EU VAT enabled" (when enabled). This setting will be disabled by deault. And it'll be used on the registation and "customer info" pages

Forum topics:

• https://www.nopcommerce.com/en/boards/topic/99613/make-vatnumber-mandatory-from-plugin
• https://www.nopcommerce.com/en/boards/topic/54032/suggestion-make-vat-field-per-checkbox-as-a-required-field
• https://www.nopcommerce.com/en/boards/topic/13117/vat-on-registration