Refactor all the data fetched from endpoint

1. In `/api/quiz/all_quizzes/{id}/slots`, there are no answers along with the questions, which is wrong. In all_quizzes, the answer and solution should be returned.
2. the `/api/quiz/all_quizzes/`'s name would be better to change to `all-quizzes`
3. trim the unnecessary data info for non-admin users, like get rid of the `visible: true` or `created_by`, etc.
