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. 


thanks @andy-armstrong, I will use the masquerade line. I wasn't sure if that masquerade bit would survive the refactor or not, but if it will still be used I'll use that logic. thanks!