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. 


Are these properties/values (max-width and width) being copied from somewhere else? If so, it may make sense to make an extend and call it from here and the place you borrowed them from.