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

closes #2906
- 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
Merge pull request #2907 from appleYaks/shadows

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