PURPOSE
Overall cleaning of subscription and exclusion management code from portal.
SPECIFICATIONS
In this commit we rename methods generating tokens (for recipients and report
management). Otherwise it is hard to distinguish them as naming is not really
convenient to use
* _unsubscribe_token -> was actually generating token for a given recipient
on a given mailing;
* _get_unsubscribe_token -> was actually generating token for mailing
users to deactivate D+1 KPI reporting;
Hash for mail tracking is also moved in its own method to avoid having the
same code twice. Better delegate it to a tool sub method.
Task-2710804 (Mail: Clean MailThread API)
Task-2669037 (Mass Mailing: Subscription page code cleaning)
Prepares Task-2150462 (Mass Mailing: Unsubscribe flow refactoring)
Part-of: odoo/odoo#106568