Allow quiz reordering, and moving quizzes between topics

[lukas-shawford]

It should be possible to change the order in which quizzes appear within a topic using drag and drop. It should also be possible to drag a quiz from one topic to another.

The backend implementation for this has been started in commit ce850e6, which contains the schema changes and a pre-save hook that would accomplish these via the Quiz.position and Quiz.topic "properties". The next step would be to add some routes to expose these properties via the API. However, rather than having separate routes for these properties, I'd like to use a single route for updating quizzes via a PATCH request, similar to how we update topic position (see commits a440989 and 9de6ccb).

However, if we go that route, then we should support PATCH for other quiz properties, including individual questions within the quiz. Thus, completion of this feature is dependent on fixing #5.