Add shadows to PostsListView and the adjacent HTML Preview; Fix preview

closes #2906, closes #2904
- add a utility function to add classnames to targets, retrofitted from clientold to accommodate `Ember.run.bind`
- create a content-preview view that tracks its scrolling
- add a scroll handler to the existing PostsListView
- The `posts/post` template now takes its HTML from the post model instead of using the editor's markdown component

I've also got this working in the editor, but this will set the stage for that when the editor PR is finished.