[FIX] account: re-introduce get_formview_id for invoices

`get_formview_id` returns a view id to open
the document with. This method is meant to be
overridden in addons that want to give
specific view ids for example.

A link to an invoice must open a different form view
according if this is a customer or a vendor invoice.

This is handled by overriding `get_formview_id`
and returning the view id that should be used
according to something in the record
(e.g. according to the `type` field).

It was present in 8.0, it was somehow
removed in Odoo 9.0 during the refactoring
of the accounting modules, probably
for a wrong reason.

opw-673246

addons/account/models/account_invoice.py

@@ -497,6 +497,14 @@ class AccountInvoice(models.Model):
         self.create_workflow()
         return True
 
+    @api.multi
+    def get_formview_id(self):
+        """ Update form view id of action to open the invoice """
+        if self.type == 'in_invoice':
+            return self.env.ref('account.invoice_supplier_form').id
+        else:
+            return self.env.ref('account.invoice_form').id
+
     @api.multi
     def get_taxes_values(self):
         tax_grouped = {}