Create Response controller

[avishkar58]

Create a controller to handle saving and submitting of Responses as documented in the application process spec.

The controller should contain

• A GET method that retrieves a response for a given event_id for the logged-in user. A serialized JSON response is returned of the form illustrated below.
• A POST method that accepts a JSON serialized response and saves the response to the database. This will make use of the data model created in Create data model for Response and Answer #21. If a response already exists (the response_id is populated in the JSON received), it should be updated, else a new response should be inserted and the new ID returned.
• A DELETE method that accepts a response_id and sets the is_withdrawn flag and the withdrawn_timestamp on the Response object

Definition of Done
This task is done when

• A new controller exists in api/app/response/api.py implementing methods described above which communicate using JSON of the form illustrated below
• Unit tests exist for the response api
• Checks exist in the API to ensure that a user can only retrieve/update/delete their own responses

An example of the JSON serialization of the Response/Answer model (NOTE: The response_id field should only be populated if the user is updating their response)
{
"response_id": 1,
"answers": [
{"question_id": 1, "value": "answer 1"},
{"question_id": 2, "value": "stuff"),
{"question_id": 3, "value": "hello world")
],
"is_submitted": false
}