Add multiple choice question schema

Route the submit method appropriately based on question type. No ability yet to save multiple choice questions.
Add validation for all question types
Get rid of question validation for now (problematic with autosave)

The issue is as soon as you click Add Question, it triggers autosave, and with the validation in place, the save fails.

I would need to implement some sort of "question draft" functionality first. You should still be able to save questions that don't have all the fields completed, but they shouldn't show up when taking the quiz. In the editor, they show up as a "draft" (i.e., there's some visual indication that the question is not complete, and that it's not going to show up when taking the quiz).

Until this happens, we either can't validate questions, or the way saving works would need to change (get rid of autosave), which I would rather not do.
Use findOne/save instead of findOneAndUpdate when saving quizzes, and improve display of error messages

The new way of saving quizzes ensures that any validation checks actually run (validation/middleware are bypassed with findOneAndUpdate).There aren't any validation checks at the moment (I just removed them in the previous commit), but if there were any, we would want them to run when saving a quiz.