Used case:

If you add a user which has no acces on a model(ex:purchase.order)
as follower on a record of this model. When someone responded by email on
this record, and when a message is sent on this record, an exception is raised
at the connection of the user added as a follower.

Fix:

To have the rights to read the message, a read notification for this follower must
be added to all parents of this message.

Closes #11902

opw:676699