[FIX] website_slides[_survey]: fix various fullscreen issues

This commit provides a few fixes for the fullscreen view:
  * make external links associated with a slide clickable again in fullscreen
    mode;
  * hide completion icons if not member:
  * add margin to subquiz icon in header;
  * add dedicated button to exit the fullscreen mode;
  * change "Pass certification" button to "Test Certification" if not member
    of the channel (for website publishers);

Commit linked to task ID 1941250 and PR #31977