[FIX] mail: correctly handle replies to thread containing external emails

When Odoo sends emails their Message-Ids are either generated by Odoo or
use the original Message-Id in the case of an incoming email going through
the mail gateway.

When the mailgateway handles replies to those emails it checks their
References or In-Reply-To in order to find matching Message-Ids and find
the reply's parent. Currently it is limited to Message-Ids matching the Odoo
pattern. However if an external email began a discussion thread, references
will not hold any odoo-like pattern.

Previously to commit 0028c421 this was hidden due to a bug when
managing private messages. This code was also managing replies to external
emails due to a probably undesired side effect.

Correct way of checking if an incoming email is a reply to a thread is to
check that References or In-Reply-To Message-Ids match some stored Message-Ids
of mail.message. They don't have to match Odoo patterN