Make the post versions more like the old format

- "Current" is now most like the old format
-- It is therefore now the default for post versions
- Only show the actual edits in their own column
- Show the current state at that version in another column
- On the "previous" view, don't double-show full list of tags for
  the first post versions, so leave edits blank

app/controllers/application_controller.rb

@@ -42,8 +42,8 @@ class ApplicationController < ActionController::Base
     super
   end
 
-  def set_version_comparison
-    params[:type] = %w[previous subsequent current].include?(params[:type]) ? params[:type] : "previous"
+  def set_version_comparison(default_type = "previous")
+    params[:type] = %w[previous subsequent current].include?(params[:type]) ? params[:type] : default_type
   end
 
   def model_name