FetchNewStories#new_stories should stop scanning when story id is matched instead of url

This is a replacement of #250.

When reading reading through feedzirra's source I found out that they already to the same kind of logic: https://github.com/pauldix/feedzirra/blob/master/lib/feedzirra/feed_entry_utilities.rb#L22

So I'd use that to check if the story was already imported. I don't think it's useful to add tests with stories that have the same URL but different id's since we stub the whole feedzirra part.

You might also want to check https://github.com/pauldix/feedzirra/pull/169. `ItunesRssItem` did not have the `entry_id` method. I am seeing some errors in my logs since some feeds seem to be parsed as ItunesRssItems.