Add a preview button to the admin interface for notes/request categories

[HelenWDTK]

We have rich text support now, which is great. It'd be good to be able to see how content will be displayed before it goes live on the site, so we can check things are displaying as I would expect. I'd like a preview button that renders the note/topic page content in the admin interface so I can see it and edit it without needing to click away and come back.