[FIX] website_slides: fix slides_tour

Before this commit, the main website_slides tour, slides_tour, was
broken after [1] introduced the WebsitePreview client action.

This commit adapts it to work with an iframe.

[1]: https://github.com/odoo/odoo/commit/31cc10b91dc7762e23b4bde9b945be0c4ce3fe3b

task-3035156

Part-of: odoo/odoo#104039