WIP: by default show only feeds with unread entries; hide feeds without unread entries.

Some acceptance tests get broken with this change, some have been fixed but not all.

Not yet implemented: add button in the sidebar to load feeds without unread entries.
Fixed acceptance tests.
FeedsController can optionally return all feeds, not just those with unread entries, if the "include_read" request parameter is passed with value "true".
Added buttons to show/hide feeds without unread entries in the sidebar.