Support single and double quotes in xblock string field editor

Addresses STUD-1984. @cahrens @andy-armstrong please review.
I think you just moved this code that someone else (probably me) wrote, but I've taken to not quoting names in JSON strings as that is unnecessary. It also makes it stand out a bit more clearly in PyCharm as being valid JSON. Would you mind changing this to:

    {display_name: updatedDisplayName}