[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/contract.py

@@ -91,6 +91,8 @@ class ContractContract(models.Model):
         string='Commercial Entity',
         index=True
     )
+    tag_ids = fields.Many2many(comodel_name="contract.tag", string="Tags")
+    note = fields.Text(string="Notes")
 
     @api.multi
     def _inverse_partner_id(self):