[FIX] website_slides: allow unsanitized html in the course description

Steps to reproduce
==================

- Install website_slides
- Go to eLearning > Basics of Gardening
- Enter edit mode
- Drop the tabs section from the right to the purple header
- Save
-> We can't switch tabs

Cause of the issue
==================

The `description` and `description_short` were converted from Text
field to Html fields in 6d914c005183aa5ae69650e3002a5a85ca83b4a6

Solution
========

Set the `sanitize_form` and `sanitize_attributes` to False

See odoo/odoo#47318

opw-2950825

closes odoo/odoo#106759

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>