Add tests that check the behavior of unfollowing a record from the inbox and
the unfollow link in email.
To support unfollowing a document in the inbox no matter the current company,
we have modified message_unsubscribe in mail_thread to allow internal user to
unsubscribe themself without checking any rights. Indeed, some document have
record rule that prevents reading it if the user is not in the right company
(ex. crm.lead) and then was preventing the user to unfollow the document using
that method. We also add a test checking that internal user can unsubscribe a
record without any read and write rights. And that a portal user can't in the
same circumstance.
Task-3061864
Part-of: odoo/odoo#107978
Pierre-Yves Dufays
a0cb558048