Purpose of this commit is to improve tracking of opened traces. Notably
a token is added to ensure we do not mess with traces and have unique
tracking URLs.
MIGRATION REMARK
Emails sent before the migration will not be marked as opened anymore after
migration. We recommend to avoid sending statistically important mass mailings
about one week before migrating database. Indeed statistics show that most of
open emails happen within the first week after being sent.
Task 2223146
closesodoo/odoo#49139
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>