[htmlparser] rename SaxOnlyValidator to XMLValidator

[htmlparser] add missing deprecation message

[form] remove hard coded id html id should be unique. It is clearly not achieved here.

[server/test] use server config in migration This allows a lighter initialisation (no rtag needed). Execution of those test move from 300 secondes to 220 seconds here. A 25% speedup

[session] deprecate `hijack_user` method The semantic of this method is wicked and lead to very hard violation of sanity. (multiple transaction using the very same cursors). We deprecated the API to be able to drop it as soon as possible. The method was added long ago for some notification trick. It is not needed since we sent notification on ``postcommit_event``. (closes #2781782)

[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)

remove unused import pyflakes say they are unused. Tests suite confirms.

one more merge