[REF] refactor the code in order to split it in several file

mail_usability/models/mail_template.py

@@ -0,0 +1,12 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 Akretion (http://www.akretion.com).
+# @author Sébastien BEAU <sebastien.beau@akretion.com>
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
+
+from openerp import fields, models
+
+
+class MailTemplate(models.Model):
+    _inherit = 'mail.template'
+
+    auto_delete = fields.Boolean(default=False)