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/artist_versions_controller.rb

@@ -2,6 +2,7 @@ class ArtistVersionsController < ApplicationController
   respond_to :html, :xml, :json
 
   def index
+    set_version_comparison
     @artist_versions = ArtistVersion.paginated_search(params)
     @artist_versions = @artist_versions.includes(:updater, artist: :urls) if request.format.html?