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
closes#2414
- Add PostsController with loadNextPage action
- Collect and store pagination info from PostsRoute into PostsController
- Use `store.filter` on PostsRoute model hook to auto-update posts list template as post models are added to the store
- Add content list view and use it to check for scroll event
- Make PostRoute only load a post that's already in the store, until we figure how to load multiple pages on refresh
- Add util function from clientold that concats error messages from server response
