Add warning for missing introduction slides to the Quiz Linter tool

This branch adds a warning to the Quiz Linter tool if a quiz does not have explanation slides before the first question in a Quiz. This should help organizers in having a more convenient quiz where they have time to start the presentation before the first question actually appears.

This MR also makes the Quiz Linter modal a little bit more generic, no hardcoded checks for certain problems anymore. Also, the panels in the modal are now built more dynamically.

This is a direct implementation of #12 (closed)