Fixed bug in User.unsubscribe.

When a folder is deleted because the user unsubscribed from the only feed in it, the returned folder id was being referenced with the wrong variable name.
Added acceptance test to ensure that after refreshing a single feed, only unread entries are shown by default.
Moved the responsibility of retrieving entries for a folder to the new User.unread_folder_entries method. Now only unread entries are shown by default.

This simplifies FoldersController#show a lot.