Support single and double quotes in xblock string field editor

Addresses STUD-1984. @cahrens @andy-armstrong please review.
I admit that I'm still wrapping my head around fixtures. Are you referring to the fact that it also installs an HTML fixture? Perhaps "install" is the wrong terminology. I'd also like to move that snippet of HTML into a file somewhere. I was looking at other specs, and it looks like `cms/templates/js/mock` directory is where they go?

On a related note: thus far I've manually made sure the `fieldEditorView.model` has a `display_name` that matches the text of the `.title-value` span, since in production, both of these values are set by the server to match the value of the xblock's data field. Should I find a way to parameterize the span's text so that the container fixture dynamically matches the model's display name?