Support single and double quotes in xblock string field editor

Addresses STUD-1984. @cahrens @andy-armstrong please review.
I agree that that was the idea when the test was in the container. Now you are testing the view itself which is the wrapper, so having a method to get a hold of the root element from the root view seems unnecessarily confusing. Anywhere you want this, you can just do: ```fieldEditorView.$el``` rather than ```getDisplayNameWrapper()``` which is just as concise and seems clearer.