Fix #4901: Duplicate disapprovals

* Add uniqueness constraint on post_disapprovals (user_id, post_id).
* Add fix script to remove existing duplicates.

script/fixes/081_delete_duplicate_post_disapprovals.rb

@@ -0,0 +1,7 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+PostDisapproval.transaction do
+  PostDisapproval.destroy_duplicates!(:user_id, :post_id)
+end