We now return generated records when calling ``_action_send_mail`` on mail
composer. This allows to have access to generated emails (in mass mode) or
posted messages (in comment mode). Otherwise we have to access sub-fields
like activity_ids / message_ids, which may be inefficient or generate extra
ACLs checks.
Also replace some |= with += when usage of OrderedSet is not necessary. To
keep ordering, ordered sets are used when doing an or between record sets.
However in some cases we know we are simply adding records not already in
a previous record set, meaning we can just use +.
Task-2883589 (Activity performance and cleaning)
Part-of: odoo/odoo#93682
Thibault Delavallée
6bea3c7605