[FIX] mass_mailing: Confirmation when sending a mass mailing

When a user clicks on the `Send to all` button in a mass mailing form,
then all emails (sometime thousands) are sent inmediatly.

With this fix, a confirmation dialog appears
and the user is asked to confirm this operation.

This simple fix prevents common human errors.

opw-659117
Closes #9796

addons/mass_mailing/views/mass_mailing.xml

@@ -281,10 +281,10 @@
                             string="Test Mailing"/>
                         <button name="send_mail" type="object"
                             attrs="{'invisible': ['|', ('state', '=', 'done'), ('body_html', '=', False)]}"
-                            class="oe_highlight" string="Send to All"/>
+                            class="oe_highlight" string="Send to All" confirm="This will send the email to all recipients. Do you still want to proceed ?"/>
                         <button name="send_mail" type="object"
                             attrs="{'invisible': ['&amp;', ('state', '!=', 'done'), ('body_html', '!=', False)]}"
-                            string="Send to All"/>
+                            string="Send to All" confirm="This will send the email to all recipients. Do you still want to proceed ?"/>
                         <field name="state" widget="statusbar"/>
                     </header>
                     <div class="oe_form_box_info oe_text_center" attrs="{'invisible': [('scheduled', '=', 0)]}">