[IMP] various: support multi-emails in '_message_post_after_hook'

PURPOSE

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

SPECIFICATIONS

Post-message hook is used in various apps to link records to a newly created
created partner. This is the case notably when used with a template as it
creates partner on the fly based on emails to always handle partners.

We now check either the complete 'email', either the normalized version of
it to avoid comparison issues with multi emails and formatted emails. As
'email_normalized' now supports multi-emails by storing the first found one
it helps finding the partner.

Task-2612945 (Mail: Defensive email formatting)

Part-of: odoo/odoo#74474