pundit: convert post versions to pundit.

app/policies/post_version_policy.rb

@@ -0,0 +1,9 @@
+class PostVersionPolicy < ApplicationPolicy
+  def undo?
+    unbanned? && record.version > 1 && record.post.present? && policy(record.post).visible?
+  end
+
+  def can_mass_undo?
+    user.is_builder?
+  end
+end