[FIX] account: reorder inherit to be able to pass through portal before mail thread

Purpose is to enable override of mail.thread methods in portal mixin.
An example is the _notification_recipients method defined in mail.thread
and customized in portal.mixin.

addons/account/models/account_invoice.py

@@ -41,7 +41,7 @@ MAGIC_COLUMNS = ('id', 'create_uid', 'create_date', 'write_uid', 'write_date')
 
 class AccountInvoice(models.Model):
     _name = "account.invoice"
-    _inherit = ['mail.thread', 'mail.activity.mixin', 'portal.mixin']
+    _inherit = ['portal.mixin', 'mail.thread', 'mail.activity.mixin']
     _description = "Invoice"
     _order = "date_invoice desc, number desc, id desc"