Purpose
=======
Allow users to re-use a design they made in a previous mailing.
Specifications
==============
Add a duplicate button when the mailing has been sent (so the users
can easily duplicate an existing mailing to re-use the template).
Allow the user to add a mailing to the favorites. When we open the web
editor for the first time, show the email bodies of the favorite
mailings. When the users click on them, load the HTML like we do for
the themes.
Task-2593231
Part-of: odoo/odoo#79174
std-odoo
5508ca04e1