Feature: Demo quiz

This feature will add a demo quiz. It will use the file "/public/test_data/quiz_with_3_questions.json" which used to be the data for the DAG Workshop. It will rename the hashtag of the quiz to "Demo Quiz XYZ" where XYZ is the current number of already existing demo quizzes on the server incremented by 1.

Unlike mentioned in #321 (closed) there is no extra button which will add the quiz - instead the user has to enter "Demo Quiz" in the hashtag field and click on "make a new one" so the user can see how to use the app. The user will also be redirected to the question route instead of the hashtagmanagement view. I would appreciate feedback if this is the way to go or if there should be a button and a route to the hashtag management.

Fixes #321 (closed) Fixes #364 (closed)