Don't link to null projects

If you have fewer than 10 projects, then empty running projects were
linked to with the path `/projects/-1`.

Copy/pasted the list strategy from the other project sections in the
same partial so the duplication becomes explicit.

app/models/project.rb

@@ -145,6 +145,10 @@ class Project < ActiveRecord::Base
     @new_record_before_save
   end
 
+  def age_in_days
+    @age_in_days ||= (Date.today - created_at.to_date + 1).to_i
+  end
+
 end
 
 class NullProject