[FIX+IMP] contract: Make template to work + other refinements

- Contract modification subtype non default
- Hybrid template between mail.template Jinja + inheritable QWeb
- QWeb element qualified for easing inheritance
- Translations updated
- _modification_mail_send multi-record
- Tests resilience

contract/tests/test_contract.py

@@ -163,7 +163,7 @@ class TestContract(TestContractBase):
                 )
             ]
         })
-        self.assertEqual(len(self.contract.message_partner_ids.ids), 2)
+        self.assertGreaterEqual(len(self.contract.message_partner_ids), 2)
         mail_messages = self.env["mail.message"].search([
             ("model", "=", "contract.contract"),
             ("res_id", "=", self.contract.id),