Exercise

Addition of the exercise generation system.
So I wanted to do polymorphic associations, but sequelize doesn't support that. I think I'd feel better if instead of storing a boolean, correct and incorrect, that we instead store a JSON blob instead. We could even store that on the Question level. So the blob on the Question that we store would look something like this:
````
answer: {
  type: 'multiple',
  values: [
    { id: 1, text: 'This is the first option', isCorrect: true }...
  ]
}

or

answer: {
  type: 'boolean',
  values: [{ id: 1, value: true }, {id: 2, value: false, isCorrect: true}]
}
````
So here we need no joins on the Question table, we have maximum flexibility going forward for new question types, and still only need store an Id on the result, even for multiple choice.