Steps to reproduce:

- Install `mass_mailing` module
- Create a new mailing
- Set a subject and a mailing list
- Select for the mail body the template "Thank you for joining us!'
- Add a dynamic placeholder (e.g. ${object.email})
- Click on "Send"
- Open the email received and click on "View Online"

Issue:

  The dynamic placeholder is not replaced.

Cause:

  The controller does not re-render the body based on the res_id.

Solution:

  Use `mailing` variable (that inherit from `mail.render.mixin`) to
  re-render the body based on the res_id if available (same as when
  sending the mail).
  https://github.com/odoo/odoo/blob/d3a6a20788c78ddaaff0919ea627e7b10c7ab81c/addons/mail/wizard/mail_compose_message.py#L476



opw-3133975

closes odoo/odoo#118996

X-original-commit: 6a0850b5
Signed-off-by: Nasreddin Boulif (bon) <bon@odoo.com>
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>