[FIX] [IMP] mail: fixed mail sent to users being redirected to their partner profile

[FIX] [IMP] mail: fixed mail sent to users being redirected to their partner profile
leading to potential issues.

Now the behavior is back to its previous version, posting on the user creates a private
discussion. Fixed management of model variable in message_post to avoid posting messages
on the mail.thread model.

Classic inbox writes on partner, with hr it writes on the employee profile, like previously.