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
Some opportunity for reuse here. One one level, you're just cycling through three actions on the same endpoint, and it would probably look better as a loop. Stepping back a bit, some way to build these URLs more automatically would help it out a lot. This could mean using something builtin (I come from Django-land and it has this, not sure if Flask does), or just reusing concepts like 'good question id'. Refactoring out the version number would help a lot as well.