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
Totally agree -- this is exactly the kind single-developer-myopia stuff I was hoping you'd catch!