[FIX] sign_oca: Define the correct url to be set in the email that is…

… sent to the signers so that everyone can sign

Example use case:
- Create a request
- Define 2 different signers
- Click on the Send button
- 2 emails will be sent and each one will have the correct url in the Sign document button

Related to OCA#71

sign_oca/models/sign_oca_request.py

@@ -393,6 +393,7 @@ def _compute_is_allow_signature(self):
                 not item.signed_on and item.partner_id == user.partner_id
             )
 
+    @api.depends("access_token")
     def _compute_access_url(self):
         result = super()._compute_access_url()
         for record in self: