[IMP] various: support multi-emails in mailings

PURPOSE

Be defensive when dealing with email fields, notably when having multi-emails
or email field containing an already-formatted email.

SPECIFICATIONS: MAIL COMPOSER IN MAILING

When using the composer with a mailing, it currently skips recipients whose
email is a multi-email due to the strict usage of 'email_normalize'.

We can improve multi-email support by effectively checking for the first
email found, using the "less strict" mode of normalize. It means more emails
are detected as valid, and therefore sent.

Due to lower support of multi-emails when sending emails, this even allows
to send multiple emails as all emails are mailed.

SPECIFICATIONS: DEFAULT RECIPIENTS

Mailings are generally done using default recipients, aka using a model method
that returns the people to mail: customers ('partner_id'), customer emails
('email_from'), specific implementation, ...

This is implementation using '_message_get_default_recipients' that returns
'partner_ids', 'email_to' and 'email_cc' that are then used in the mail
composer to generate final recipients.

In this commit we better handle the content of email fields to avoid issues
with multi-emails. For that purpose we correctly split the content of those
fields. We now have several 'email_to' for records having multi-emails instead
of a single badly-formatted 'email_to'.

Task-2612945 (Mail: Defensive email formatting)

Part-of: odoo/odoo#74474