Encapsulate the concept of staleness into its own class

Extract it from a single helper method and wrap it in a method object.
This lets us add tests around it and then (later) split the domain
concerns from the view concerns.

lib/staleness.rb

@@ -0,0 +1,18 @@
+require 'active_support/all'
+
+class Staleness
+  def self.days_stale(item, current_user)
+    if item.due || item.completed?
+      return ""
+    elsif item.created_at < current_user.time - (current_user.prefs.staleness_starts * 3).days
+      return " stale_l3"
+    elsif item.created_at < current_user.time - (current_user.prefs.staleness_starts * 2).days
+      return " stale_l2"
+    elsif item.created_at < current_user.time - (current_user.prefs.staleness_starts).days
+      return " stale_l1"
+    else
+      return ""
+    end
+  end
+end
+