[FIX] website_slides: add various design and UX fixes

- slidesChannelDropdownSort : dropdown items back to visible (was white on white)
- Featured lesson : change icon to bullhorn instead of + icon
- Pass certification fullscreen button : btn primary
- set max height to fullscreen content to 100% minus header
- Fix 'infographic' slide type image view
- change image for 'Furniture Technical Specifications' course
- fix fullscreen presentation mode: The condition to add the iframe that
  contains the presentation of the document was wrong and, so, never met.
  Those conditions are evaluated in javascript and not in python as it's part
  of t-name template linked to a javascript widget.

Commit linked to task ID 1941250 and PR #31279.