overdue_reminder: link mails to res.partner

Advantages:
1) easy access to the history in the chatter of the partner.
2) no problem to access the email by other users (avoid "The requested
   operation cannot be completed due to security restrictions. Please
   contact your system administrator.")

Migration script provided

account_invoice_overdue_reminder/__manifest__.py

@@ -5,7 +5,7 @@
 
 {
     'name': 'Overdue Invoice Reminder',
-    'version': '10.0.2.0.1',
+    'version': '10.0.2.0.2',
     'category': 'Accounting',
     'license': 'AGPL-3',
     'summary': 'Simple mail/letter/phone overdue customer invoice reminder ',

account_invoice_overdue_reminder/migrations/10.0.2.0.2/pre-migration.py

@@ -0,0 +1,17 @@
+# Copyright 2020 Akretion France (http://www.akretion.com/)
+# @author: Alexis de Lattre <alexis.delattre@akretion.com>
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
+
+
+def migrate(cr, version):
+    if not version:
+        return
+
+    cr.execute("""
+        UPDATE mail_message SET
+        res_id=overdue_reminder_action.commercial_partner_id,
+        model='res.partner'
+        FROM overdue_reminder_action, mail_mail
+        WHERE overdue_reminder_action.mail_id=mail_mail.id
+        AND mail_mail.mail_message_id = mail_message.id
+        """)

account_invoice_overdue_reminder/wizard/overdue_reminder_wizard.py

@@ -442,8 +442,8 @@ def validate_mail(self):
             'subject': self.mail_subject,
             'body_html': self.mail_body,
             'email_cc': ', '.join(cc_list),
-            'model': False,  # don't link to the wizard !
-            'res_id': False,
+            'model': 'res.partner',
+            'res_id': self.commercial_partner_id.id,
             })
         mvals.pop('attachment_ids', None)
         mvals.pop('attachments', None)