[FIX] /web/binary/{image,saveas}: use request.registry instead of deprecated `request.session.model`

This makes the controllers work also for
public users, e.g. in the frontend.
[FIX] website_forum: avoid access error when closing spam/abuse questions

Logging the user login is not necessary, but
accessing it might cause access errors. Logging
the UID is equivalent anyway, so let's not bother.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64