[FIX] website_sale: better fiscal position detection + support update during checkout

[FIX] website_sale: better fiscal position detection + support update during checkout

The customer can change the country and tax
number in the billing information during
checkout, and the taxes should be properly
updated according to the re-detected fiscal
position.

The fiscal position detection also depends
on the `vat_subjected` flag, which we now assume
to be implicit as soon as the customer filled
in a valid Tax Identification Number.