[FIX] mass_mailing : restore import mass_mailing.contact into mass_mailing.list

Since the opt out per mailing list, the intermediate model was used
to list all the recipient of a mailing list. The side effect was that
importing new contacts into mailing list was not possible anymore.
Also, creation of a new contact into a mailing list was made indirectly
by creating a new many2many record between contaxct and list, which was
not really user friendly.

This commit restores the mailing contact model as the view of the mailing
list recipients. In other terms, the contact import is restored.
An opt-out computed field as been added to be able to filter on opted out
contact inside the mailing list recipients view.
This opt_out field SHOULD NOT be used in a form view as it need the active
mailing list id in the context in order to be computable and searchable.
This field has no sense without this active mailing list id context.

Task ID 1891228
Linked with blacklist and opt out per mailing task (ID 33224)
Closes PR #27632