Avoid calling thrice the same function during the rendering of the
template.
`can_edit_vat()` checks whether partner has invoices or sales orders to
avoid VAT modifications, but this check should only be done once by
rendering.
Gain of up to 8 queries (if sale & account installed).
X-original-commit: 3860749074608068efdba940ccb983d89516cf8d
Victor Feyens
9004e90ab0