Use fetched results controller to load locally saved feeds

Now that FeedManager doesn't return cached feeds anymore,
we let the fetched results controller do that task for us. By not
waiting until the latest feed arrives from a remote server, we
can present the users with previously fetched feeds immediately
when they go to the feed list view.
Inject FeedBuilder as a dependency to FeedManager
Remove data manager as feed manager's dependency

Now that the fetched results controller is in action and the
feed builder is responsible for saving the new feed into
the database, feed manager doesn't need a feeds data
manager anymore.