Limit unread date and stories for performance

Some users never use mark all read. To support their use case, move up
the unread date if it's too old. This shortens the incoming list for the
queries so they don't take too long or lots of resources. Next, limit the
number of stories to something sane that angular can cope with. Do this by
taking the X most recent stories. In both cases, we need to make sure the
read stories list only contains stories we care about (i.e., nothing from
the past or culled list).