Add alternate comparison types to versions

- The types are:
-- Previous: The default and the previously used type
-- Subsequent: Compares against the next version
-- Current: Compares against the current version
- Allow switching between comparison types in index and diff views
-- Have links vary depending upon current comparison type

app/controllers/application_controller.rb

@@ -39,6 +39,10 @@ class ApplicationController < ActionController::Base
     super
   end
 
+  def set_version_comparison
+    params[:type] = %w[previous subsequent current].include?(params[:type]) ? params[:type] : "previous"
+  end
+
   def model_name
     controller_name.classify
   end