[FW][FIX] mass_mailing: can unsubscribe in multidomain

[fw-bot]

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 is no longer
found and replaced into the source.

opw-2124890

Forward-Port-Of: #42373

[fw-bot]

Ping @nle-odoo
Cherrypicking 12125e3 of source #42373 failed

stderr:

error: could not apply 12125e31a1f... [FIX] mass_mailing: can unsubscribe in multidomain
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add <paths>' or 'git rm <paths>'
hint: and commit the result with 'git commit'

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

[nle-odoo]

should be fixed, I will just test on runbot before merging

[nle-odoo]

robodoo r+