#721: fix markup bug for clickable labels

This closes #721.

@matthewrmshin, please review.

Testing instructions:

Open the `demo/rose-config-edit/demo_meta/app/02-help` app, navigate to the `Env section title` page and deselect the menu option `Metadata -> Layout Preferences -> Hide Variable Help`. This code won't give a warning and fail to construct the label, but the head of master will.