fix initial story hotness

Initial story hotness was zero, which excluded stories with no other
upvotes from the homepage.  Before creating, define initial hotness
to something.

Since this now makes very new stories show up on the homepage right
away, expand the window back to 48 hours.

This requires a Story.recalculate_all_hotnesses! to properly sort
things.

app/models/story.rb

@@ -36,6 +36,7 @@ class Story < ActiveRecord::Base
 
   before_validation :assign_short_id_and_upvote,
     :on => :create
+  before_create :assign_initial_hotness
   before_save :log_moderation
   after_create :mark_submitter, :record_initial_upvote
   after_save :update_merged_into_story_comments
@@ -126,6 +127,10 @@ class Story < ActiveRecord::Base
     h
   end
 
+  def assign_initial_hotness
+    self.hotness = self.calculated_hotness
+  end
+
   def assign_short_id_and_upvote
     self.short_id = ShortId.new(self.class).generate
     self.upvotes = 1
@@ -154,9 +159,10 @@ class Story < ActiveRecord::Base
     end
 
     # TODO: as the site grows, shrink this down to 12 or so.
-    window = 60 * 60 * 24
+    window = 60 * 60 * 48
 
-    return -((order * sign) + base + (self.created_at.to_f / window)).round(7)
+    return -((order * sign) + base +
+      ((self.created_at || Time.now).to_f / window)).round(7)
   end
 
   def can_be_seen_by_user?(user)