FIX: subscriptions use vat instead email

FIX Issue #26 (closed)

https://trello.com/c/LGRZ3Bli/159-use-vat-to-identify-existing-respartner-instead-of-email

Change list:

• Inherit method of cooperator to search by vat instead of email.
• Added method to partner to search by vat.
• Added VAT field required and states to fix control in form view.

TODO

• company_register_number is a field used by cooperator for cooperators companies, we should use vat for companies and persons.
• Create a PR to l10n_es_cooperator, we're still waiting OCA to resolve the PR