[FIX] mass_mailing: can unsubscribe in multidomain

The `unsubscribe` feature of mass mailings repose on having a link with
HREF attribute `/unsubscribe_from_list` inside the mail message.

When the mass mailing is sent:

- relative URL are replaced by absolute URL (`/unsubscribe_from_list` is
  replaced by `{system parameter web.base.url}/unsubscribe_from_list`)

- `{system parameter web.base.url}/unsubscribe_from_list` is replaced by
  the real mass mailing link containing info that will be used to
  unsubscribe the user.

But there was an issue in the case of multiple domain, if this scenario
happened:

- system parameter web.base.url is http://domain1
- a user use "Test" button on a mass mailing
- system parameter web.base.url becomes http://domain2


- the mass mailing is sent

The unsubscribe link is broken, this is because the implementation of
"Test Mailing" feature would update the mass mailing with absolute
links, so if the domain change, we the `unsubscribe` link will no longer
be found and replaced into the source.

opw-2124890
closes #42373

closes odoo/odoo#43277

X-original-commit: e5311b9ddb89b223486a8e1050e5c7a2d017ea07
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>