[IMP+FIX] contract: Several things

- Add failing test for next invoice date before the last date invoiced
- raise an error when next invoice date before the last date invoiced
- Add note field to contract
- add new option: create_new_line_at_contract_line_renew
  Add a company config option to decide whether to create or to extend contract
  line at renew action
- extend contract line at renewal
- improve code: unify methods argument _renew_create_line and _renew_extend_line

contract/models/__init__.py

@@ -9,3 +9,6 @@ from . import contract_line
 from . import account_invoice
 from . import account_invoice_line
 from . import res_partner
+from . import contract_tag
+from . import res_company
+from . import res_config_settings