Infinite Scroll on Posts List

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