[REF] mail: remove private discussion support in mail gateway

PURPOSE

Add some improvements in mail gateway: remove private discussion, improve
bounce management, allow resetting bounce counters, improve automatic set or
reset of blacklists and ease mass mailing inheritance.

SPECIFICATIONS

Private discussion support in mail gateway means having incoming messages
not linked to a document. It means having a void model and res_id on related
messages.

Old private conversations in Odoo were implemented using messages not linked
to any document [1]. It was possible to send an email to some people through
the UX and they were able to answer it. Messages were not attached to any
document and model. There were some limitations to this feature, notably it was
limited to replies (no beginning of private discussion through mail gateway).
One way of doing it was through the use of res.users aliases [2]. There was
also a vaguely twitter-lite use of chatter on HR employee profiles linked
to users aliases, then removed with an explicit support of private
discussions [3].

It is not supported anymore since discussion between users is now done using
chat. Aliases on users have since been removed [4]. Contacting partners or
customers can be done on business documents or on channels. Simple email
discussion is not supported in Odoo as it is not the purpose of mailgateway.

This commit removes code related to private discussion in mail gateway and
some specific support in message_post method.

LINKS

Related to task 1893155
Linked to PR #33340

[1] History is a bit messy, see notably fd90140d7bd05820aeac7bd7e22539893f87319d and commits around
[2] User alias addition 052f2ace57e20f06980bc8fadf37b32be1921420
[3] End of employee twitter-like and posting on users = private discussion 89896f32bbf6aeecfbddc0f7e8397781fce2a99c
[4] User alias removal 029d1baf359eabb7ed531287f3cb2b3b8d586d39

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