Support single and double quotes in xblock string field editor

Addresses STUD-1984. @cahrens @andy-armstrong please review.
I'm not sure if I understand what you mean in this comment. Are you saying we should just use the $el directly? I think the idea behind this helper method was that the markup could change in the future, and we'd (hopefully) only need to change this method, rather than every test, to account for that.