[FIX] web: force seession context reload when reloading client

The goal is to ensure that HTTP requests
done while reloading the client (e.g. the
menu bar, etc.) do not use a deprecated
session context.

Also undo the previous fix from a0ee2b5
as it would cause other issues (e.g.
prevent setting the admin password/lang
when creating a new db using the db manager)
This logic does not belong in the business
methods of res.users.
[ADD,IMP]:add a python test case in website_forum