lms preview mode toggle improvement

@talbs, @explorerleslie - Here's a branch for the lms preview toggle. The bar shows up when the user is an instructor, but this isn't wired up to toggle the view based on the active element in the select field yet. Similarly, the existing Staff View/Student View toggle hasn't been removed yet. 


This label looks to be visually hidden and for screen readers. Does it need vertical-alignment and margin in that case?