[mail] drop hijack_user usage The transaction is now commited before notification. We can create a new session directly and drop `hijack_user` usage. This prepare #2781782

[notification] merge BaseNotificationView and NotificationView They are in the same module now.

[notification] move notification view in ``sobject.notification`` It has no user outside this module. This enforce serversideness of notification and allow future cleanup. No backward compat is set up to prevent circular import. The class has no other user anyway. (closes 2845144)