[FIX] website_slides: fix ui for quiz creation and update on mobile

Before, the buttons to save, cancel or update quiz in eLearning frontend
as well as icons weren't displayed correctly.

Now, all elements are displayed correctly on mobile or on computer, more
responsive and better displayed.

Task 2153990
PR #42846

closes odoo/odoo#43328

X-original-commit: 4d986beb7192c34c133d8d5d02ff62bd159f202a
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>