Optional validation of VAT via VIES

This module extends the standard VAT validation so that you can see, on each contact, whether the VAT number was actually accepted by the European VIES service.

• On the contact form, right after the VAT field, a read-only "VIES validation" checkbox shows the result. It is filled automatically when the VAT number or country changes, and when the VAT number is saved.
• If the contact is a child of a company, it inherits the VIES result of its parent company instead of being checked on its own.
• In Settings, under the VIES service option, a "Compute check vies" button lets you run a bulk check on all company contacts that have a VAT number and no VIES result yet. A confirmation warning appears first because it can take a long time, and a summary message shows how many contacts were processed and how many failed.
• Unlike the standard behaviour, a VAT number that is valid in its own country is not blocked just because VIES did not confirm it; the flag simply stays unchecked.

The module touches res.partner and res.config.settings, and adds fields to the contact form and the settings screen.