Serialize answer dict before calculating size for datadog

When sending the submission answer size to DataDog, I was taking the length of `submission['answer']`, forgetting that this is now a JSON-serializable dict with a `text` key.  Since the length of a dictionary is the number of keys, the answer size was always reported as one.

This change serializes the answer to a JSON string, then sends the length of the string to DataDog.