[FIX] website_forum, website_slides: correctly check if is admin

Before this commit and since 2f7c03d9, the admin user has not ID=1
anymore.

Thus, some old condition were not doing what they were expected to.

Firstly, the admin was not considered as such on the forum, and if its karma
points were not high enough, it would not be possible to do anything for the
admin. Note that it was fixed in saas-12.1 with 1dac4131

Secondly, some _search functions would not return a truthy value anymore.

closes odoo/odoo#33359

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>