[FIX] website_slides: fix various quiz-related issues

This commit provides a few fixes for quiz feature:

  * fix amount of karma won by the user in a quiz;
  * correctly display the fact that a subquiz was completed by the user instead
    of just looking like a non-completed quiz;
  * going on a previously completed quiz, good answers are hightlighted and
    inputs disabled;
  * effectively put the quiz in readonly mode if not a channel member.

Commit linked to task ID 1941250 and PR #31977