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 needs to be adjusted because in the case of the instructor bar NOT being there, there is more space below the navigation tabs than above.