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/post_versions_controller.rb

@@ -6,6 +6,7 @@ class PostVersionsController < ApplicationController
   respond_to :js, only: [:undo]
 
   def index
+    set_version_comparison
     @post_versions = PostVersion.paginated_search(params)
 
     if request.format.html?