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. 


Also, I'd recommend making this element's UI-based class name a bit more general. Something that pertains to the overall preview-purpose of the UI might be good. Something like ``wrapper-preview-menu``.