Update exercise json

- [x] remove `parts:[]`
- [x] rename `part.background` to `question.stimulus`

Example JSON:

```json
{
    "type": "exercise",
    "content": {
      "background":"The spaceship moves at <span data-math='\\dfrac{-b \\pm \\sqrt{b^2 - 4ac}}{2a}'></span> 1 m/s",
      "questions":[
        {
          "id": "123",
          "format": "short-answer",
          "stem":"What is the rest mass in kg?"
        },{
          "id": "234",
          "format": "multiple-choice",
          "stem":"What is the force if it slams into a wall?",
          "answers":[
            {"id":"id1","value":"10","content":"10 N"},
            {"id":"id2","value":"1","content":"1 N"}
          ]
        }
      ]
    }
  }
```