[IMP] website_slides: allow user to download slides pdf by default

PURPOSE

To align on the v13.0 enable the 'Allow Download' option by default.

SPECIFICATION

Change the default value of slide_resource_downloadable from False
to True.

LINKS

Task ID : 2225802
PR : #49951

closes odoo/odoo#50164

X-original-commit: 44bfb8c73f9abc563ce764411610f45e5bdf18b1
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>