In which we allow users to submit answers to questions

- Add basic CRUD for creating answers to questions
- Answers can either be text or the url of an uploaded file, but they must match the question type of their parent question
- The question type of a question can not be changed after it is created
- Answer have a question_type field that always matches the question_type of their parent question

@wilhow @maiamcguinness @malynda @thcrock
Yeah, flask definitely has 
```python
url_for(Answer, question_id=1, answer_id=1)
```
which you'll see me use everywhere except the unit tests because I sure must be a crazy person who loves refactoring unit tests when the URL's change!