Add the ability to copy-paste item_data strings from /api/v1

Summary:
If the JSON in the json editor is actually a string, we call
JSON.parse on that string. This means we can copy the extra
jsonified strings from /api/v1/assessment_items into test.html,
and they'll magically show up as json! :D

Test Plan:
copy paste the item_data string from
https://www.khanacademy.org/api/v1/assessment_items/x83227c8991a45ecc?version=edit

Auditors: joel, justinjaffray