Improve quiz editing API to allow more granular saving

[lukas-shawford]

Currently, editing quiz questions results in re-saving the entire quiz from scratch (including all questions) any time a change is made. This is not very efficient, especially with auto-save. The backend should be rewritten to add support for more granular saving (probably using PATCH requests, like with topics). If only one question got changed, then ideally the request body should only include that question, and only that one question should be rewritten when saving to the database. While we're at it, might as well make the API for editing quizzes/questions adhere to REST principles.