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/helpers/artist_commentary_versions_helper.rb

@@ -0,0 +1,10 @@
+module ArtistCommentaryVersionsHelper
+  def commentary_version_field_diff(commentary_version, type, field)
+    other = commentary_version.send(params[:type])
+    if type == "previous"
+      diff_body_html(commentary_version, other, field)
+    else
+      diff_body_html(other, commentary_version, field)
+    end
+  end
+end