Only load posts once on navigating to content tab

No Issue
- Loading posts from the API should not be necessary in PostsIndexRoute
  because its parent resource (PostsRoute) pre-loads the store.
  Changing the store.find to store.all gets rid of a duplicate
  network request to load all posts.
Merge pull request #3347 from jaswilli/posts-content

Only load posts once on navigating to content tab