[IMP] account: reconciliation models: introduce partner mapping

Partner mapping is a new field defined on invoice matching reconciliation models. It allows defining a mapping between partners and regular expressions. When ran, on a statement line without any partner set, those rules use this mapping to assign a partner to statement lines without any one set, using the regular expressions to match the payment reference of the statement line.

It is important to note that the mapped partner will only be set to the statement line when the reconciliation is actually performed (so, when clicking the validate button, if the matching rule is not auto_reconcile = True), just like with the partner_map.