[FIX] website_slides: do not display links to unviewable slides

Channel allow to browse slides and a list of available slide is displayed
as a left panel in the eLearning frontend. Even slides current user
cannot see are displayed in order to give some information about course
content. Accessible slides are clickable, other slides should be
displayed as strings. However due to some sudo when computing slide
informations all links are set as clickable.

There is no security issue as when being redirected to a slide the
user cannot access there is an acl error. However he should not have
the links displayed, just strings. This commit fixes that behavior.

closes odoo/odoo#42338

X-original-commit: 04a957bb
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>