Fix #4901: Duplicate disapprovals

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

test/unit/application_record.rb

@@ -45,4 +45,20 @@ class ApplicationRecordTest < ActiveSupport::TestCase
       end
     end
   end
+
+  context "ApplicationRecord#destroy_duplicates!" do
+    should "destroy all duplicates" do
+      @post1 = create(:post, score: 42)
+      @post2 = create(:post, score: 42)
+      @post3 = create(:post, score: 42)
+      @post4 = create(:post, score: 23)
+
+      Post.destroy_duplicates!(:score)
+
+      assert_equal(true,  Post.exists?(@post1.id))
+      assert_equal(false, Post.exists?(@post2.id))
+      assert_equal(false, Post.exists?(@post3.id))
+      assert_equal(true,  Post.exists?(@post4.id))
+    end
+  end
 end