[FIX] website_slides: build a correct form in Courses

How to reproduce the problem:
- Install the website_slides modules
- Go to the website -> Courses -> choose a course -> Add Content (Webpage)
- In the new Webpage -> Edit -> Drag and drop a Form Builder -> Save
The form is unusable: you can't write in the form's fields.

Cause of the problem : usefull attributes and values of the form were
removed due to the sanitizing of the HTML component.

Solution : the HTML components in slide are not sanitized anymore,
in order to keep valuable information for building forms.

opw-2588089

closes odoo/odoo#78138

X-original-commit: f783e238
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>
Signed-off-by: prro-odoo <proose@users.noreply.github.com>