[FIX] website_slides: correct auto_delete on mail

Steps to reproduce
==================
1. Open course with enroll policy onInvitation or
   Click on three dot of any course in kanban view
2. Click on Invite > add a recipient
3. Go to mail template internal link 
4. Go to setting tab
5. Disable the 'Auto delete' > save & close it 
6. Click on Send
7. Go the Technical/Emails (in debug mode)
8. Send the mail by clicking 'Send now'
Issue - Even if we unchecked auto delete still the mail is deleted automatically

Technical
=========
In the '_prepare_mail_values' method of the 'slide.channel.invite' model, the
mail values for 'mail.mail' are prepared and the auto delete is set to True.

After this commit
=================
You can view the sent mails since the auto_delete is configured based on
the template value for invitation.

Task-3546212

closes odoo/odoo#141141

X-original-commit: 3d9640d5c24f345d67e4e74e62584e60cc27815b
Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>

addons/website_slides/wizard/slide_channel_invite.py

@@ -92,7 +92,7 @@ class SlideChannelInvite(models.TransientModel):
         mail_values = {
             'attachment_ids': [(4, att.id) for att in self.attachment_ids],
             'author_id': self.env.user.partner_id.id,
-            'auto_delete': True,
+            'auto_delete': self.template_id.auto_delete if self.template_id else True,
             'body_html': body,
             'email_from': self.env.user.email_formatted,
             'model': None,